Cardiac resynchronization therapy diagnostics

ABSTRACT

In some examples, a system can be used for delivering cardiac resynchronization therapy (CRT). The system may include a pacing device configured to be implanted within a patient. The pacing device can include a plurality of electrodes, signal generation circuitry configured to deliver ventricular pacing via the plurality of electrodes, and a sensor configured to produce a signal that indicates mechanical activity of the heart. Processing circuitry can be configured to identify one or more features of a cardiac contraction within the signal, and determine whether the contraction was a fusion beat based on the one or more features.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/615,703, entitled, “CARDIAC RESYNCHRONIZATIONTHERAPY DIAGNOSTICS,” and filed on Jan. 10, 2018, the entire content ofwhich is incorporated by reference.

TECHNICAL FIELD

The disclosure relates to medical devices, and more particularly medicaldevices that monitor physiological conditions and deliver pacingtherapy.

BACKGROUND

Some types of implantable medical devices, such as cardiac pacemakers orimplantable cardioverter defibrillators, provide therapeutic electricalsignals to a heart of a patient, such as bradycardia pacing, cardiacresynchronization therapy (CRT), anti-tachycardia pacing (ATP), andcardioversion/defibrillation shocks. The therapeutic electrical signalsmay be delivered to the heart in the form of pulses or shocks forpacing, cardioversion or defibrillation. In some cases, an implantablemedical device may sense intrinsic depolarizations of the heart, andcontrol the delivery of therapeutic signals to the heart based on thesensing.

CRT is one type of electrical stimulation therapy delivered by animplantable medical device. CRT may help enhance cardiac output byresynchronizing the electromechanical activity of the ventricles of theheart. Ventricular desynchrony may occur in patients that suffer fromcongestive heart failure (CHF). CRT includes delivering pacing stimulito both ventricles (sometimes referred to as biventricular pacing), orto one ventricle (e.g., fusion pacing, such as left-ventricular pacing)with the intended result of a substantially simultaneous mechanicalcontraction and ejection of blood from the ventricles. Pacing may bedelivered in the right ventricle (RV) and/or the left ventricle (LV) torestore ventricular synchrony.

Achieving a positive clinical benefit from CRT may be dependent onseveral therapy control parameters, such as the timing intervals used tocontrol pacing pulse delivery to one or both ventricles, e.g., anatrio-ventricular (A-V) interval and/or an inter-ventricular (V-V)interval. The A-V interval controls the timing of ventricular pacingpulses relative to a preceding atrial event, e.g., an intrinsic or paceddepolarization. The V-V interval controls the timing of a pacing pulsein one ventricle relative to a paced or intrinsic event in the otherventricle. Other therapy control parameters include a CRT pacingconfiguration, e.g., a fusion (single ventricle) configuration or abiventricular configuration, and a selection of electrodes used todeliver the ventricular pacing to a particular ventricle when more thanone electrode is available and their polarities. In some cases, a leadmay include multiple, e.g., four, electrodes from which one or more maybe selected. In some cases, multiple electrodes may be selected ascathodes for a given chamber.

Medical device technology advancement has led toward smaller and smallerimplantable devices. Recently, cardiac pacemakers have been introducedwhich can be implanted directly in a heart chamber. In some examples,such pacemakers may be leadless and delivered into the heart chamberusing a catheter. Such miniaturized pacemakers may be referred to asintracardiac pacing devices (PDs), although they may be epicardially orextracardially implanted in some examples. An intracardiac PD may beconfigured to deliver CRT, e.g., as part of a system with one or moreother devices.

SUMMARY

In general, this disclosure is directed to techniques for CRT. Moreparticularly, this disclosure is directed to techniques for evaluatingCRT, e.g., determining a percentage or other amount of CRT pacing pulsesthat effectively capture the heart, based on an evaluation of thecontraction following the CRT pacing pulse delivery. In some examples,the contraction evaluation is performed, at least in part, by anintracardiac PD including a motion sensor that generates a signal thatvaries with cardiac contraction, e.g., one or more accelerometers. Insome examples, the amount of effective CRT, or other metrics of CRTeffectiveness, are reported to a user, e.g., via a computing device incommunication with the intracardiac PD. Based on the contractionevaluation, processing circuitry may determine, for example, whether thecontraction was a fusion beat.

A fusion beat is typically characterized by a wave complex formed bydepolarization of the myocardium initiated by two different foci orsources, commonly a non-native stimulus from a PD and a native stimulus.A fusion beat happens when the depolarization from the different sourcesoccur simultaneously (or nearly simultaneously) at the same region ofthe heart. There may be degrees of fusion, e.g., based on the degree ofsimultaneity, and a fusion beat as used herein may refer to a beat inwhich the degree of fusion was sufficient to be characterized as fusion,e.g., some metric of fusion was greater than a threshold.

In one example, a system for determining the effectiveness of cardiacresynchronization therapy (CRT) comprises a pacing device configured tobe implanted within a patient. The pacing device comprises a pluralityof electrodes, signal generation circuitry configured to deliverventricular pacing pulses for the cardiac resynchronization via theplurality of electrodes, and a sensor configured to produce a signalthat indicates mechanical activity of the heart. The system furthercomprises processing circuitry configured to, for each of a plurality ofcardiac cycles during which the pacing device delivered one of theventricular pacing pulses, identify one or more features of a cardiaccontraction within the signal during the cardiac cycle, for each of theplurality of cardiac cycles, determine whether the cardiac contractionis a fusion beat based on the one or more features, and determine avalue of at least one metric indicating the effectiveness of the CRTbased on the determinations of whether the cardiac contractions arefusion beats. The system further comprises a user interface configuredto present the value of the at least one metric to a user.

Another example is a method for determining the effectiveness of cardiacresynchronization therapy (CRT) by a pacing device. The methodcomprises, by processing circuitry of a medical device system comprisingthe pacing device, receiving a signal from a sensor of the pacingdevice, the signal indicating mechanical activity of a heart, for eachof a plurality of cardiac cycles during which the pacing devicedelivered a ventricular pacing pulse for the CRT, identifying one ormore features of a cardiac contraction within the signal during thecardiac cycle, for each of the plurality of cardiac cycles, determiningwhether the cardiac contraction is a fusion beat based on the one ormore features, determining a value of at least one metric indicating theeffectiveness of the CRT based on the determinations of whether thecardiac contractions are fusion beats, and presenting the value of theat least one metric to a user via a user interface.

Other examples include a system comprising means for receiving a signalfrom a sensor of the pacing device, the signal indicating mechanicalactivity of a heart, for each of a plurality of cardiac cycles duringwhich the pacing device delivered a ventricular pacing pulse for theCRT, means for identifying one or more features of a cardiac contractionwithin the signal during the cardiac cycle, for each of the plurality ofcardiac cycles, means for determining whether the cardiac contraction isa fusion beat based on the one or more features, means for determining avalue of at least one metric indicating the effectiveness of the CRTbased on the determinations of whether the cardiac contractions arefusion beats, and means for presenting the value of the at least onemetric to a user via a user interface.

Other examples include a computer-readable media comprising instructionsthat, when executed by processing circuitry of a medical device system,cause the processing circuitry to receive a signal from a sensor of thepacing device, the signal indicating mechanical activity of a heart, foreach of a plurality of cardiac cycles during which the pacing devicedelivered a ventricular pacing pulse for the CRT, identify one or morefeatures of a cardiac contraction within the signal during the cardiaccycle, for each of the plurality of cardiac cycles, determine whetherthe cardiac contraction is a fusion beat based on the one or morefeatures, determine a value of at least one metric indicating theeffectiveness of the CRT based on the determinations of whether thecardiac contractions are fusion beats, and present the value of the atleast one metric to a user via a user interface.

Other examples include a system for determining the effectiveness ofcardiac resynchronization therapy (CRT), the system comprises a pacingdevice configured to be implanted within a patient, the pacing devicecomprises a plurality of electrodes, a signal generation circuitryconfigured to deliver ventricular pacing pulses for the cardiacresynchronization via the plurality of electrodes to a left ventricle, athree-dimensional accelerometer configured to produce a signal thatindicates mechanical activity of the heart, a housing configured forimplantation on or within the heart, wherein the signal generationcircuitry and the sensor are within the housing. The system furthercomprises processing circuitry configured to for each of a plurality ofcardiac cycles during which the pacing device delivered one of theventricular pacing pulses, identify one or more features of a cardiaccontraction within the signal during the cardiac cycle, for each of theplurality of cardiac cycles, determine whether the cardiac contractionis a fusion beat based on the one or more features, and determine avalue of at least one metric indicating the effectiveness of the CRTbased on the determinations of whether the cardiac contractions arefusion beats. The system further comprises an external device configuredto communicate with the pacing device, the external device comprises auser interface configured to present the value of the at least onemetric to a user, wherein the processing circuitry comprises processingcircuitry of the external device and processing circuitry of pacingdevice, and wherein the one or more features of the cardiac contractionwithin the signal comprise an amount of motion in at least one directionother than the primary axis of motion during the cardiac contraction.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the methods and systems described in detailwithin the accompanying drawings and description below. The details ofone or more aspects of the disclosure are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

The details of one or more examples of this disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of this disclosure will be apparent from thedescription and drawings, and from the claims.

FIG. 1A is a conceptual diagram illustrating an example front view of apatient implanted with an example medical device system that includes anextracardiovascular ICD (EV-ICD) system and a pacing device (PD) that isimplanted within a cardiac chamber of the patient in accordance with oneor more aspects of this disclosure.

FIG. 1B is a conceptual diagram illustrating an example side view of apatient implanted with the example medical device system of FIG. 1A, inaccordance with one or more aspects of this disclosure.

FIG. 1C is a conceptual diagram illustrating an example transverse viewof a patient implanted with the example medical device system of FIG.1A, in accordance with one or more aspects of this disclosure.

FIG. 2 is a conceptual diagram illustrating an example front view of apatient implanted with another example medical device system thatincludes an insertable cardiac monitoring (ICM) device that is insertedsubcutaneously or substernally in the patient, and a PD implanted withina cardiac chamber of the patient, in accordance with one or more aspectsof this disclosure.

FIG. 3 is a conceptual drawing illustrating an example configuration ofthe ICM device illustrated in FIG. 2, in accordance with one or moreaspects of this disclosure.

FIG. 4 is a conceptual drawing illustrating an example configuration ofa PD in accordance with one or more aspects of this disclosure.

FIG. 5 is a functional block diagram illustrating an exampleconfiguration of an IMD in accordance with one or more aspects of thisdisclosure.

FIG. 6 is a functional block diagram illustrating an exampleconfiguration of a PD in accordance with one or more aspects of thisdisclosure.

FIG. 7 is a functional block diagram illustrating an exampleconfiguration of the external device in FIG. 1A, in accordance with oneor more aspects of this disclosure.

FIG. 8 is a block diagram illustrating a system that includes anexternal device, such as a server, and one or more computing devicesthat are coupled to an IMD, PD, and external device via a network, inaccordance with one or more aspects of this disclosure.

FIG. 9 is a flow diagram illustrating an example process delivering,evaluating, and adjusting cardiac resynchronization therapy through aPD, in accordance with one or more aspects of this disclosure.

FIG. 10 is a flow diagram illustrating an example process fordetermining parameter values for cardiac resynchronization therapy basedon an evaluation of one or more cardiac contraction features, inaccordance with one or more aspects of this disclosure.

FIG. 11A is a plot illustrating cardiac motion during normal sinusrhythm (NSR, intrinsic beat) looking down the systolic axis, inaccordance with one or more aspects of this disclosure.

FIG. 11B is a plot illustrating cardiac motion during right ventricular(RV) pacing looking down the systolic axis, in accordance with one ormore aspects of this disclosure.

FIG. 12 is a plot of cardiac motion illustrating example features of acardiac motion signal, in accordance with one or more aspects of thisdisclosure.

DETAILED DESCRIPTION

In general, this disclosure describes example techniques related todelivery of CRT to a patient, e.g., using an intracardiac PD. Theintroduction of such PDs, and the resulting elimination of the need fortransvenous intracardiac leads, provides several advantages. Forexample, complications due to infection associated with a lead extendingfrom a subcutaneous pacemaker pocket transvenously into the heart can beeliminated. Other complications, such as “twiddler's syndrome,” leadfracture, or poor connection of the lead to the pacemaker are eliminatedin the use of an intracardiac PD.

In some examples, a ventricular PD may be unable to detect atrialdepolarizations. In some examples, another implantable medical device(IMD) detects atrial depolarizations. The other IMD may send signals tothe ventricular PD. The signals may indicate the timing of atrialdepolarizations, or may be a trigger to deliver a ventricular pacingpulse.

One or both of the ventricular PD or the other IMD may maintain timingintervals that control the timing of ventricular pacing pulses for CRTrelative to atrial events, such as paced or intrinsic depolarizations.According to some example techniques for delivering CRT, a memory, e.g.,of a ventricular PD that delivers CRT, another IMD, and/or an externalcomputing device, stores one or more values of one or more intervalsbetween an atrial event and a ventricular event, e.g., an A-V interval,determined for the patient. An atrial or ventricular event may be adepolarization or contraction, as examples, but may be any fiducialmarker in an EGM or other signal that varies with cardiac electrical ormechanical activity. In some examples, the ventricular event is a sensedor intrinsic ventricular event, and the interval is an A-V_(s) interval.In some examples, the memory stores values for both of an intervalbetween a sensed atrial event and the ventricular event, e.g., anA_(s)-V_(s) interval, and an interval between a paced atrial event andthe ventricular event, e.g., an A_(p)-V_(s) interval. The values storedfor the patient in the memory may be determined by monitoring the A-Vconduction of the patient prior to, or during a suspension of, deliveryof CRT.

Processing circuitry, e.g., of the ventricular PD, the other IMD, and/orthe external computing device, may control the delivery of CRT based ona selected value of the A-V interval. For example, the processingcircuitry may determine whether to control a ventricular PD to deliverfusion pacing in which only one ventricle is paced in coordination withthe other ventricle's intrinsic activation, e.g., left-ventricularpacing, or two PDs to deliver biventricular pacing to the patient basedon the selected A-V interval value. In examples in which the A-Vinterval value selected based on the heart rate is an A-V_(s) intervalvalue, the processing circuitry may further determine, based on theselected A-V_(s) value, an A-V_(p) interval value for timing thedelivery of the fusion or biventricular pacing. Additionally, oralternatively, a PD or other IMD may similarly maintain and utilize V-Vintervals that control the timing of ventricular pacing pulses for CRTin one ventricle relative to ventricular events in another ventricle,such as paced or intrinsic depolarizations of the other ventricle.

CRT may be made more effective for a given patient by determiningpatient specific values of CRT control parameters, such as A-V and/orV-V interval values. Effective CRT for a given patient may be maintainedby periodically updating to determine parameter CRT control values. Leftventricular hemodynamic variables, blood pressure measures, orechocardiography have been used to determine and/or update CRT controlparameter values for a patient. However, such metrics are not readilyevaluated in a closed loop manner by the implantable medical device thatdelivers the CRT.

According to some techniques for adapting delivery of CRT, the storedA-V or V-V interval values may be validated and, if necessary, updatedperiodically, by suspending the delivery of CRT, and measuring a currentvalue of the A-V or V-V interval (representative of the currentintrinsic A-V conduction) while delivery of CRT is suspended. Suspendingthe delivery of CRT may refer to, as examples, withholding ventricularpacing for one or more cardiac cycles, increasing an A-V_(p) delaysufficiently so that intrinsic A-V conduction may be observed, or pacingone ventricle at a sufficiently long A-V_(p) delay and measuringintrinsic conduction on the other ventricle. The stored value of the A-Vor V-V interval may be updated if the currently measured value of theinterval to is not sufficiently similar to the stored value, e.g., byreplacing the stored A-V or V-V interval value with the currentlymeasured value.

In addition to periodic adjustment of A-V and/or V-V intervals based onobservation of intrinsic conduction, some medical devices may beconfigured to automatically switch the pacing configuration, e.g., froma fusion pacing configuration to a biventricular pacing configuration(or vice versa), based on periodic intrinsic conduction observations.For example, medical devices configured to provide AdaptivCRT™,available from Medtronic plc of Dublin, Ireland, are configured to bothautomatically change CRT control intervals and CRT pacing configurationbased on periodic observations of intrinsic conduction to achieve moreefficient physiologic pacing and to improve hemodynamics of the patient.Fusion pacing and biventricular pacing are described in further detailbelow. While the pacing stimuli may be pacing pulses or continuous timesignals, the pacing stimuli are primarily referred to herein as pacingpulses for ease of description. An example of the AdaptivCRT™ algorithmis described in U.S. Pat. No. 9,403,019 to Sambelashvili et al., whichis entitled, “ADAPTIVE CARDIAC RESYNCHRONIZATION THERAPY,” and issued onAug. 2, 2016. U.S. Pat. No. 9,403,019 to Sambelashvili et al. isincorporated herein by reference in its entirety. Another example ofAdaptivCRT™ is described in U.S. Pat. No. 9,789,319 to Sambelashvili,which is entitled, SYSTEMS AND METHODS FOR LEADLESS CARDIACRESYNCHRONIZATION THERAPY,” and issued on Oct. 17, 2017. U.S. Pat. No.9,789,319 to Sambelashvili is incorporated herein by reference in itsentirety.

Fusion-based CRT (also referred to herein as fusion pacing) may beuseful for restoring a depolarization sequence of a heart of a patient,which may be irregular due to ventricular dysfunction, in patients withpreserved intrinsic atrial-ventricular (AV) conduction. In a fusionpacing configuration, a medical device delivers one or more fusionpacing pulses to one of the ventricles, and not the other. The medicaldevice delivers the one or more fusion pacing pulses to alater-contracting ventricle (V2) to pre-excite the V2 and synchronizethe depolarization of the V2 with the depolarization of the earliercontracting ventricle (V1). The paced ventricular activation of the V2may “fuse” (or “merge”) with the ventricular activation of the V1 thatis attributable to intrinsic conduction of the heart. In this way, theintrinsic and pacing-induced excitation wave fronts may fuse togethersuch that the depolarization of the V2 is resynchronized with thedepolarization of the V1.

The medical device may be configured to deliver the fusion pacing pulseto the V2 according to a fusion pacing interval, which indicates thetime relative to an atrial pace or sense event at which the fusionpacing pulse should be delivered to the V2. An atrial sense event maybe, for example, a P wave of a sensed electrical cardiac signal and anatrial pacing event may be, for example, the time at which a stimulus isdelivered to the atrium.

In some examples, the right ventricle (RV) may be the V1 and the leftventricle (LV) may be the V2. In other examples, the LV may be the V1while the RV may be the V2. While the disclosure primarily refers toexamples in which the first depolarizing ventricle V1 is the RV and thelater depolarizing ventricle V2 is the LV, the devices, systems,techniques described herein for providing CRT may also apply to examplesin which the first depolarizing ventricle V1 is the LV and the laterdepolarizing ventricle V2 is the RV.

In some fusion pacing techniques, a pacing pulse to the V2 (V2_(P)) isdelivered upon expiration of a fusion pacing interval that is determinedbased on the intrinsic depolarization of the V1, which may be indicatedby a sensing of ventricular activation (V1s). Ventricular activation maybe indicated by, for example, an R-wave of a sensed electrical cardiacsignal. An example of a fusion pacing technique that times the deliveryof the V2 pacing pulse (V2_(P)) to the intrinsic depolarization of theV1 is described in U.S. Pat. No. 7,181,286 to Burnes et al., which isentitled, “APPARATUS AND METHODS OF ENERGY EFFICIENT, ATRIAL-BASEDBI-VENTRICULAR FUSION-PACING,” and issued on Feb. 20, 2007. U.S. Pat.No. 7,181,286 to Burnes et al. is incorporated herein by reference inits entirety.

In one example disclosed by U.S. Pat. No. 7,181,286 to Burnes et al., apacing pulse to the V2 (V2_(P)) is delivered a predetermined periodfollowing an atrial pace or sense event (A_(P/S)), where thepredetermined period is substantially equal to the duration of timebetween the atrial pace or sense event (A_(P/S)) and a V1 sensing event(V1_(s)) of at least one prior cardiac cycle decremented by a durationof time referred to as the pre-excitation interval (PEI). Thus, oneexample equation that may be used to determine a fusion pacing interval(A_(P/S)-V2_(P)):

A _(P/S)-V2_(P)=(A _(P/S)-V1_(S))−PEI   Equation (1)

A cardiac cycle may include, for example, the time between the beginningof one heart beat to the next heartbeat. The duration of time betweenthe atrial pace or sense event (A_(P/S)) and a V1 sensing event (V1_(S))may be, for example, a measurement of intrinsic AV conduction time froman atrium to the first contracting ventricle of the heart of thepatient. The PEI may indicate the amount of time with which a V2 pacingpulse precedes a V1 sensing event to achieve the fusing of theelectromechanical performance of the V1 and V2. That is, the PEI mayindicate the amount of time from the delivery of the V2 pacing pulsethat is required to pre-excite the V2, such that the electromechanicalperformance of V1 and V2 merge into a fusion event. In some examples,the PEI is automatically determined by a medical device delivering thepacing therapy, e.g., based on determined intrinsic conduction times,while in other examples, the PEI may be predetermined by a clinician. Insome examples, the PEI is a programmed value (e.g., about onemillisecond (ms) to about 250 ms or more, such as about 100 ms to about200 ms, or about 10 ms to about 40 ms) or is an adaptive value, such asabout 10% of a measured intrinsic A-V2 conduction interval or measuredintrinsic A-A cycle length.

The magnitude of the PEI may differ based on various factors, such asthe heart rate of the patient, a dynamic physiologic conduction statusof the heart of the patient, which may change based upon thephysiological condition of the patient (e.g., ischemia status,myocardial infarction status, and so forth), as well as factors relatedto the therapy system, such as the location of sensing electrodes of thetherapy system, the location of the pacing electrodes of the therapysystem, and internal circuitry processing delays of the medical device.

Another technique for determining the timing of the delivery of a pacingpulse to a later depolarizing ventricle (V2) (which is sometimes alsoreferred to as a “fusion pacing interval”) is described in U.S. PatentApplication Publication No. 2010/0198291 by Sambelashvili et al., whichis incorporated herein by reference in its entirety. In some examplesdescribed by U.S. Patent Application Publication No. 2010/0198291 bySambelashvili et al., the timing of the delivery of a pacing pulse isbased on a depolarization of the V2 in at least one prior cardiac cycle.The depolarization of the V2 may be detected by sensing an event in theV2 (V2_(S)), such as an R-wave of an electrical cardiac signal. The V2pacing pulse (V2_(P)) is timed such that an evoked depolarization of theV2 is affected in fusion with the intrinsic depolarization of the firstdepolarizing ventricle (V1), resulting in a ventricularresynchronization. In this way, the V2 pacing pulse (V2_(P)) maypre-excite the conduction delayed V2 and help fuse the activation of theV2 with the activation of the V1 from intrinsic conduction. The intervalof time between the V2 pacing pulse (V2_(P)) and the V2 sensing event(V2_(S)) of the same cardiac cycle may be referred to as the adjustedPEI.

In some examples disclosed by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al., the predetermined period at whichan IMD delivers the V2 pacing pulse (V2_(P)) following an atrial pace orsense event (A_(P/S)) is substantially equal to the duration of timebetween an atrial event (sensed or paced) (A_(P/S)) and a V2 sensingevent (V2_(S)) of at least one prior cardiac cycle decremented by aduration of time referred to as an adjusted PEI. That is, in someexamples, the adjusted PEI indicates the desired interval of timebetween the delivery of the V2 pacing pulse (V2_(P)) and the V2 sensingevent (V2_(S)) of the same cardiac cycle. One example equation that maybe used to determine the timing of a fusion pacing pulse using atechnique described by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al. is:

A _(P/S)-V2_(P)=(A_(P/S)-V2_(S))−adjusted PEI   Equation (2)

The duration of time between an atrial pace or sense event (A_(P/S)) anda V2 sensing event (V2_(S)) may be referred to as an atrioventricular(A-V) interval or delay. The adjusted PEI may indicate an interval oftime prior to a V2 sensing event (V2_(S)) at which it may be desirableto deliver the V2 pacing pulse (V2_(P)) to pre-excite the V2 and mergethe electromechanical performance of V2 and V1 into a fusion event. Insome examples described by U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al., an adjusted PEI is a linearfunction that is based on V1 sensing event (V1_(S)) and a V2 sensingevent (V2_(S)) of the same cardiac cycle, based on the time between anatrial pace or sense event (A_(P/S)) and a V2 sensing event, or anycombination thereof.

As an example, adjusted PEI may be determined as follows:

Adjusted PEI=a(V1_(S)-V2_(S))+b   Equation (3)

According to U.S. Patent Application Publication No. 2010/0198291 bySambelashvili et al., in Equation (3), the coefficients “a” and “b” maybe fixed, empiric coefficients that are selected by a clinician ordetermined based on an adjusted PEI value selected by a clinician. Insome examples, the coefficient “a” in Equation (3) may be about 1 andthe coefficient “b” may be substantially equal to the PEI. In this case,the adjusted PEI is substantially equal to a time interval between a V1sensing event (V1_(S)) and a V2 sensing event (V2_(S)) of the samecardiac cycle, incremented by the PEI. As a result, the A_(P/S)-V2_(P)interval for timing the delivery of a V2 pacing pulse may be determinedas follows:

A_(P/S)-V2_(P)=(A _(P/S)-V2_(S))−[(V1_(S)-V2_(S))+PEI)]  Equation (4)

Other values for the “a” and “b” coefficients in Equation (3) may beselected. In addition, other types of fusion pacing configurations mayalso be used in accordance with the techniques described herein. Forexample, other fusion pacing intervals described by U.S. Pat. No.7,181,286 to Burnes et al. and U.S. Patent Application Publication No.2010/0198291 by Sambelashvili et al. can also be used to control fusionpacing in accordance with techniques described herein. An example of CRTis described in U.S. Pat. No. 6,871,096 to Hill, which is entitled“SYSTEM AND METHOD FOR BI-VENTRICULAR FUSION PACING” and is incorporatedherein by reference in its entirety.

In some examples, fusion pacing is implemented as left-ventricularpacing. Most left-ventricular pacing is via a lead introduced into thecoronary sinus such that the electrodes are located proximate theleft-ventricular free wall. Left intra-cardiac, endocardial stimulationhas been proposed to improve left ventricular electrical activation,e.g., relative to pacing using a coronary sinus lead. The highelectrical conduction velocity of the endocardium allows for a faster,more homogenous activation of the myocardium. At the same time, theoptimal stimulation location of the left ventricular stimulation sitemay become less important than with electrodes on a coronary sinus lead.An intracardiac PD implanted within the left ventricle may be configuredto provide intra-cardiac, endocardial stimulation. In other examples, anextracardiac PD may be coupled to electrodes on a lead that arepositioned to provide intra-cardiac, endocardial stimulation.

In contrast to fusion pacing, in a biventricular pacing configuration,one or more medical devices, e.g., including at least one intracardiacPD, may deliver pacing pulses to both the LV and the RV to resynchronizethe contraction of the LV and RV. In a biventricular pacingconfiguration, the medical device(s) may deliver stimulation tocoordinate contraction of the LV and the RV, even in the absence ofintrinsic AV conduction of the heart.

In some proposed pacing techniques for adapting CRT, such as AdaptivCRT,a pacing configuration (e.g., fusion pacing or biventricular pacing) andtiming of the pacing pulses (e.g., a fusion pacing interval, such as aA_(P/S)-V2_(P) interval, or biventricular pacing intervals, such as anA_(P/S)-V1_(P) and A_(P/S)-V2_(P) intervals, or an A_(P/S)-V1_(P) andV1_(P)-V2_(P) intervals) are periodically adjusted based on periodicintrinsic conduction measurements to achieve more efficient physiologicpacing and optimal hemodynamics. For example, some proposed cardiacrhythm management medical device systems are configured to deliver CRTby delivering pacing to a heart of a patient in accordance with a fusionpacing configuration and, if loss of intrinsic AV conduction is detected(e.g., AV block), switching to a biventricular pacing configuration.Thus, a medical device system configured to adapt CRT, e.g., to performthe AdaptivCRT™ algorithm, may be configured to switch from a fusionpacing configuration to a biventricular pacing configuration in responseto determining a heart of a patient is no longer intrinsicallyconducting.

In some existing proposed techniques, a medical device system switchesfrom a fusion pacing configuration to a biventricular pacingconfiguration if the loss of intrinsic AV conduction is detected basedon a measurement of intrinsic conduction time, which may be performed aspart of the A-V interval determination. For example, loss of intrinsicAV conduction may be detected if a measured A-V1 conduction time(A_(P/S)-V1_(S)) is greater than (or greater than or equal to in someexamples) a predetermined threshold value. In some examples, thepredetermined threshold value is selected based on previous intrinsicconduction time intervals (e.g., may be a percentage of a mean or medianof a certain number of prior intrinsic conduction time measurements). Inother examples, the predetermined threshold value may be selected by aclinician to be, for example, a value that indicates the depolarizationtime of V1 that results in maintenance of ventricular synchrony orcardiac output at a desirable level.

To measure intrinsic conduction time, the CRT pacing may be suspended toallow the heart of the patient to conduct in the absence of cardiacrhythm management therapy and to avoid interference between the deliveryof pacing pulses and sensing of ventricular activation. In someexamples, if pacing is delivered to an atrium of the heart, such pacingmay be maintained, while pacing to the ventricles may be suspended. Themeasurement of intrinsic conduction time may be determined, e.g., as thetime between an atrial pace or sense event (A_(P/S)) and a V1 sensingevent (V1_(S)), which may be referred to generally as an A-V_(s)interval. In such examples, the determinations of the intrinsicconduction time measurements may take place, for example, once a minute,once an hour, or once every 24 hours, although other frequencies mayalso be used.

The determinations of intrinsic conduction time may involve thesuspension of some or all pacing therapy to the heart of the patient forat least one cardiac cycle, which may reduce the amount ofsynchronization of the ventricles of the heart during at least that onecardiac cycle. Furthermore, determinations of intrinsic conduction time,being based on electrical intra-cardiac conduction, may not account forcardiac tissue properties, such as myocardial stiffness and contractileproperties. As described herein, the devices, systems, and techniquesfor providing CRT are directed to adapting CRT control parameters, suchas timing intervals and pacing configuration, while lessening oreliminating the need to suspend the delivery of electrical stimulationto the heart of the patient and the reliance on electrical conductionmeasurements.

Further, a ventricular PD may have difficulty delivering CRT by itself.For example, a ventricular PD may have difficulty sensing a paced orintrinsic atrial depolarization from which to time the delivery of itsventricular pacing pulse, e.g., due to the relatively small distancebetween its electrodes. A ventricular PD may, for similar reasons, havedifficulty detecting whether its pacing pulses captured the ventricle.Certain existing techniques for evaluating whether ventricular pacingpulses for CRT captured the ventricle as intended utilize a far-fieldEGM to detect capture. Additionally, algorithms for adapting the timingparameters of CRT, such as those described above, typically use afar-field electrogram (EGM) to detect intrinsic AV conduction duringsuspension of CRT. A far-field EGM may not be obtainable a ventricularPD due to the relatively small distance between its electrodes.

FIGS. 1A-1C are conceptual diagrams illustrating various views of anexample cardiac medical device system 8A implanted within a patient 14.Components with like numbers in FIGS. 1A-1C may be similarly configuredand may provide similar functionality. Medical device system 8A asillustrated in FIGS. 1A-1C may be configured to perform one or more ofthe techniques described herein with respect to evaluating and adaptingCRT.

FIG. 1A is a conceptual diagram illustrating is an example front view ofa patient implanted with an example cardiac medical device system 8Athat includes an extracardiovascular implantable cardioverterdefibrillator (ICD) system 4A, and a pacing device (PD) 12A that isimplanted within a cardiac chamber of patient 14 in accordance with oneor more aspects of this disclosure. PD 12A may be, for example, animplantable leadless pacing device (e.g., a pacemaker, cardioverter,and/or defibrillator) that provides electrical signals to heart 16A viaelectrodes carried on the housing of PD 12A.

With respect to FIGS. 1A-1C, and elsewhere herein, PD 12A is generallydescribed as being attached within a chamber of heart 16A. That is, PD12A is described in various portions of this disclosure as anintracardiac pacing device. In other examples that are consistent withaspects of this disclosure, PD 12A may be attached to an externalsurface of heart 16A, such that PD 12A is disposed outside of heart 16Abut can pace a desired chamber. In one example, PD 12A is attached to anexternal surface of heart 16A, and one or more components of PD 12A maybe in contact with the epicardium of heart 16A. Although PD 12A isgenerally described as a pacing device for intracardiac implantation, PD12A may alternatively be configured to attach to an external surface ofheart 16A and operate as an extracardiac pacing device.

In one example, PD 12A can be implanted within left ventricle of a heartto sense electrical activity of heart and/or deliver electricalstimulation, e.g., CRT such as fusion pacing, to heart. Fusion pacingmay involve LV only pacing with PD 12 in coordination with the intrinsicRV activation. Alternatively, fusion pacing can involve pacing the RVwith PD 12 in coordination with the intrinsic LV activation. In thisscenario, PD 12 is placed on or within the right ventricle.

PD 12A is schematically shown in FIG. 1A attached to a wall of the leftventricle via one or more fixation elements (e.g. tines, helix, etc.)that penetrate the tissue. These fixation elements may secure PD 12A tothe cardiac tissue and retain an electrode (e.g., a cathode or an anode)in contact with the cardiac tissue. PD 12A (and PD 12B in FIG. 2) may beimplanted at or proximate to the apex of the heart. In other examples, aPD may be implanted at other left-ventricular locations, e.g., on thefree-wall or septum.

PD 12A may also include one or more motion sensors (e.g.,accelerometers, gyroscopes, or electrical or magnetic field sensors)configured to detect and/or confirm cardiac conditions (e.g.,ventricular dyssynchrony or tachyarrhythmias) from these mechanicalmotions of heart 16. The mechanical motions of the heart detected usingsuch sensors may also be used to evaluate contractions during CRTaccording to the techniques described herein. In examples in which PD12A is implanted at or near the apex of heart 16, the motion sensor maycorrespondingly be located at or near the apex. Since PD 12A includestwo or more electrodes carried on the exterior housing of PD 12A, noother leads or structures need to reside in other chambers of heart 16.However, in other examples, medical device system 8A may includeadditional PDs within respective chambers of heart 16 (e.g., leftatrium, right atrium), or coupled by leads to electrodes in suchchambers of heart.

ICD system 4A includes ICD 10A that is connected to at least oneimplantable cardiac defibrillation lead 18A (hereinafter,“defibrillation lead 18A”). ICD 10A is configured to deliver high-energycardioversion shocks or defibrillation pulses to heart 16A of patient14, in response to atrial fibrillation or ventricular fibrillation beingdetected. Cardioversion shocks are typically delivered in synchrony witha detected R-wave, when fibrillation detection criteria are met.Defibrillation pulses are typically delivered when fibrillation criteriaare met, and the R-wave cannot be discerned from signals sensed by ICD10A.

ICD 10A of FIG. 1A may be implanted subcutaneously or submuscularly onthe left side of patient 14 above the ribcage. FIG. 1C is a conceptualdiagram illustrating an example transverse view of a patient implantedwith the example medical device system of FIG. 1A, in accordance withone or more aspects of this disclosure. Defibrillation lead 18A of FIG.1A may be implanted at least partially in a substernal location in FIG.1A, e.g., between the ribcage and/or sternum 22 and heart. In one suchconfiguration, a proximal portion of defibrillation lead 18A extendssubcutaneously from ICD 10A toward the sternum, and a distal portion oflead 18A extends under or below the sternum 22 in the anteriormediastinum 36 (see FIG. 1C). The anterior mediastinum 36 is boundedlaterally by the pleurae 39 (see FIG. 1C), posteriorly by thepericardium, and anteriorly by the sternum 22. In some instances, theanterior wall of the anterior mediastinum 36 may also be formed by thetransversus thoracis and one or more costal cartilages. The anteriormediastinum 36 includes a quantity of loose connective tissue (such asareolar tissue), some lymph vessels, lymph glands, substernalmusculature (e.g., transverse thoracic muscle), branches of the internalthoracic artery, and the internal thoracic vein. In one example, thedistal portion of defibrillation lead 18A extends along the posteriorside of the sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36. Defibrillationlead 18A may be at least partially implanted in other intrathoraciclocations, e.g., other non-vascular, extra-pericardial locations,including the gap, tissue, or other anatomical features around theperimeter of and adjacent to, but not attached to, the pericardium orother portion of heart 16A and not above the sternum 22 or ribcage.

In other examples, defibrillation lead 18A may be implanted at otherextracardiovascular locations. For example, defibrillation lead 18A mayextend subcutaneously above the ribcage from ICD 10A toward a center ofthe torso of patient 14, bend or turn near the center of the torso, andextend subcutaneously superior above the ribcage and/or sternum 22, likethat shown in FIG. 1A. Defibrillation lead 18A may be offset laterallyto the left or the right of the sternum 22 or located over the sternum22. Defibrillation lead 18A may extend substantially parallel to thesternum 22 or be angled lateral from the sternum 22 at either theproximal or distal end. In another example, defibrillation lead 18Aand/or a pacing lead or sensing lead may be implanted within thepericardial sac of heart 16A, within the pericardium of heart 16A,epicardially with respect to heart 16A, or at another location.

Defibrillation lead 18A of FIG. 1A may include an insulative lead bodyhaving a proximal end that includes a connector configured to beconnected to ICD 10A and a distal portion that includes one or moreelectrodes. Defibrillation lead 18A may also include one or moreconductors that form an electrically conductive path within the leadbody and interconnect the electrical connector and respective ones ofthe electrodes.

Defibrillation lead 18A of FIG. 1A includes a defibrillation electrodethat, in the illustrated example, includes two sections or segments 20Aand 20B. Segments 20A and 20B are collectively (or alternatively)referred to herein as “defibrillation electrodes 20.” Defibrillationelectrodes 20 of FIG. 1A are positioned toward the distal portion ofdefibrillation lead 18A, e.g., toward the portion of defibrillation lead18A extending along sternum 22 of patient 14. Defibrillation lead 18A ofFIG. 1A is placed below and/or along sternum 22 such that a therapyvector between defibrillation electrodes 20A or 20B and a housingelectrode formed by ICD 10A or on ICD 10A (or other second electrode ofthe therapy vector) is substantially across a ventricle of heart 16A.The therapy vector may, in one example, be viewed as a line that extendsfrom a point on defibrillation electrodes 20 (e.g., a center of one ofthe defibrillation electrode sections 20A or 20B) to a point on thehousing electrode of ICD 10A. Each of defibrillation electrodes 20 ofFIG. 1A may, in one example, be an elongated coil electrode. In someexamples, a defibrillation lead may include more or fewer than the twodefibrillation electrodes 20 in the illustrated example ofdefibrillation lead 18A, such as a single coil defibrillation electrode20.

FIG. 1B is a conceptual diagram illustrating an example side view of apatient implanted with the example medical device system of FIG. 1A, inaccordance with one or more aspects of this disclosure. Defibrillationlead 18A may also include one or more sensing electrodes, such assensing electrodes 22A and 22B, located along the distal portion ofdefibrillation lead 18A. In the example illustrated in FIGS. 1A and 1B,sensing electrodes 22A and 22B are separated from one another bydefibrillation electrode 20A. In other examples, however, sensingelectrodes 22A and 22B may be both distal of defibrillation electrodes20, or both proximal of defibrillation electrodes 20. In other examples,defibrillation lead 18A may include a greater number or a fewer numberof electrodes at various locations proximal and/or distal todefibrillation electrodes 20. In these and/or other examples, ICD 10Amay include one or more electrodes on another lead (not shown in FIGS.1A-1C).

ICD system 4A may sense electrical signals via one or more sensingvectors that include combinations of electrodes 22A and 22B and thehousing electrode of ICD 10A. For example, ICD 10A may obtain electricalsignals that are sensed using a sensing vector between sensingelectrodes 22A and 22B, obtain electrical signals sensed using a sensingvector between sensing electrode 22B and the conductive housingelectrode of ICD 10A, obtain electrical signals sensed using a sensingvector between sensing electrode 22A and the conductive housingelectrode of ICD 10A, or a combination thereof. In some instances, ICD10A may sense cardiac electrical signals using a sensing vector thatincludes one of the defibrillation electrode sections 20A and 20B andone of sensing electrodes 22A and 22B or the housing electrode of ICD10A.

The sensed electrical intrinsic signals include electrical signals thatare generated by cardiac muscle and are indicative of depolarizationsand repolarizations of heart 16A at various times during the cardiaccycle. Moreover, the sensed electrical intrinsic signals may beindicative of one or more cardiac events with respect to the functioningof heart 16A. The sensed electrical signals may also include electricalsignals, e.g., pacing pulses, generated by PD 12A and delivered to heart16A. ICD 10A analyzes the electrical signals sensed by the one or moresensing vectors to detect tachyarrhythmia, such as atrial tachycardia,atrial fibrillation, ventricular tachycardia, or ventricularfibrillation. In response to detecting the tachyarrhythmia, e.g., aventricular fibrillation, ICD 10A may begin to charge a storage element,such as a bank of one or more capacitors. Upon determining that thestorage element is sufficiently charged, ICD 10A may deliver one or moredefibrillation pulses to certain chamber(s) of heart 16A viadefibrillation electrodes 20 of defibrillation lead 18A, if ICD 10Adetermines that the tachyarrhythmia is still present.

In the example of FIG. 1A, PD 12A is implanted within the left ventricleof heart 16A, to provide pacing pulses to the left ventricle, e.g., forCRT. While illustrated as being implanted within the left ventricle asan example, it will be appreciated that PD 12A may be implanted atdifferent positions as well. For instance, PD 12A may be implantedepicardially. That is, in accordance with epicardial implantation, PD12A may be positioned externally to heart 16A and may be connected viaone or more leads or in a leadless fashion to the left ventricle ofheart 16A. In other examples, PD 12A or other PDs may be implantedwithin or externally to other chambers of heart 16A

PD 12A may be constructed to have dimensions to fit within the availablevolume of the left ventricle of heart 16A and to be attachable to awall, e.g., at or near the apex, of the left ventricle of heart 16A. Asmaller size of PD 12A may also reduce the risk of thrombus forming inheart 16A. In some examples, PD 12A may leverage EGM sensingcapabilities of ICD 10A, and therefore, may not include EGM sensingcircuitry. As such, PD 12A may utilize a smaller capacity battery thanin scenarios where regular EGM sensing for electrical cardiac events isperformed.

For example, ICD 10A may be configured to sense electrical activity ofheart 16A, such as atrial depolarizations or P-waves, and determine whenPD 12A should deliver one or more pacing signals (e.g., pulses) to theleft ventricle of heart 16A. ICD 10A may then transmit control signalsto PD 12A to provide timing information associated with the pacingpulses that are to be delivered. The timing information may bedetermined based on one or more stored A-V or V-V intervals, which maybe determined by processing circuitry, e.g., of ICD 10A and/or PD 12A,as described above. Upon receiving the control signals from ICD 10A, PD12A may deliver the pacing signals or pulses according to the timinginformation indicated by the received control signals. ICD 10A and PD12A may operate using transmission schedules and communication schedulesto limit the amount of time that PD 12A operates communication circuitrythat receives the control signals in a powered-on state.

In some examples, ICD 10A may also provide pacing signals as part of CRTusing sensing electrodes 22A and/or 22B of defibrillation lead 18A. Inother examples, ICD 10A may be coupled to one or more intracardiac leadscarrying respective electrodes configured to be disposed within theright atrium and the right ventricle of heart 16A, and deliver pacingpulses via these intracardiac leads as part of the CRT along with PD12A. In other examples, additional PDs like PD 12A may be disposedwithin the right atrium and/or the right ventricle of heart 16A. AnyPD(s) placed within the right atrium and/or right ventricle of heart 16Amay be similarly controlled by ICM 10A. Alternatively, one or both PDsin the right atrium and/or right ventricle may provide control signalsto PD 12A disposed in the left ventricle of heart 16A.

In another example, PD 12A implanted in the left ventricle and/or a PDimplanted in the right ventricle or other heart chamber may beconfigured to deliver other pacing therapy, such as bradycardia pacingtherapy, anti-tachycardia pacing (ATP), and/or post-shock pacing, toheart 16A. For example, PD 12A or a PD implanted in or on the rightventricle may deliver A-V synchronous bradycardia pacing therapy, timedrelative to the atrial depolarization based on control signals receivedfrom ICD 10A in accordance with the techniques described herein.

Again, in some examples, PD 12A may not include EGM sensing circuitry.In other examples, PD 12A may be capable of sensing electrical signalsusing the electrodes carried on the housing of PD 12A. These electricalsignals may be electrical signals generated by cardiac muscle andindicative of depolarizations (e.g. a ventricular depolarization orR-wave, or an atrial depolarization or P-wave) and repolarizations (e.g.a ventricular repolarization or T-wave) of heart 16A at various timesduring the cardiac cycle. PD 12A may analyze the sensed electricalsignals to detect tachyarrhythmia, such as ventricular tachycardia orventricular fibrillation, bradyarrhythmia, or even shocks. In responseto detecting these conditions, PD 12A may, e.g., depending on the typeof arrhythmia or shock, begin to deliver bradycardia pacing therapy orpost-shock pacing, with or without information from another device. Insome examples, PD 12A may only detect arrhythmias in response to failingto detect control signals from ICM 10A for a predetermined period, orover a predetermined number of communication windows.

Although FIG. 1A is illustrated and described in the context of asubsternal ICD system 4A and a PD 12A, techniques in accordance with oneor more aspects of the present disclosure may be applicable to othermedical device systems. One example of another medical device system 8that may implement the techniques of this disclosure is shown in FIG. 2and discussed in further detail below with respect to FIG. 2. In anotherexample, instead of an extravascular ICD (EV-ICD) system, a subcutaneousor submuscular pacing device coupled to a ventricular intracardiac leadmay be implanted within the patient. In this manner, the pacing devicemay provide pacing pulses to the right ventricle of heart 16A via theintracardiac lead, and control PD 12A to provide pacing pulses to theleft ventricle of heart 16A. In another example, a subcutaneous orsubmuscular pacing device coupled to a ventricular intracardiac leadcarrying electrodes may be coupled to a motion sensor, e.g. by the lead,another lead, or wirelessly, and may implement the techniques of thisdisclosure for evaluating contractions during CRT. As such, in someexamples, the sensor may be included as a part of an endocardial lead,such as a left-endocardial lead. The examples of FIGS. 1A-1C and 2 arefor illustrative purposes and should not be considered limiting of thetechniques described herein, in any way.

External device 24 may be configured to communicate with ICD 10A and/orPD 12A. In examples where external device 24 only communicates with oneof ICD 10A or PD 12A, the non-communicative device may receiveinstructions from or transmit data to the device in communication withexternal device 24. In some examples, external device 24 may include,be, or be part of one or more of a handheld computing device, a computerworkstation, or a networked computing device. External device 24 mayinclude a user interface that is configured or otherwise operable toreceive input from a user. In other examples, external device 24 mayprocess user interactions that are relayed remotely, such as via anetworked computing device. External device 24 may process userinteractions to enable users to communicate with PD 12A and/or ICD 10A.For example, external device 24 may process user inputs to send aninterrogation request and retrieve therapy delivery data, to updatetherapy parameters that define therapy, to manage communication betweenPD 12A and/or ICD 10A, or to perform any other activities with respectto PD 12A and/or ICD 10A. Although the user is typically a physician,technician, surgeon, electrophysiologist, or other healthcareprofessional, the user may be patient 14 in some examples.

External device 24 may also allow the user to define how PD 12A and/orICD 10A senses electrical signals (e.g., cardiac electrograms (EGMs)),detects arrhythmias (e.g., tachyarrhythmias), delivers therapy (e.g.,CRT), and communicates with other devices of cardiac medical devicesystem 8A. For example, external device 24 may be used to changetachyarrhythmia detection parameters. In another example, externaldevice 24 may be used to manage therapy parameters. When PD 12A and ICD10A are configured to communicate with each other, external device 24may be used to alter communication protocols between PD 12A and ICD 10A.For example, external device 24 may instruct PD 12A and/or ICD 10A toswitch between one-way and two-way communication and/or change which ofPD 12A and/or ICD 10A are tasked with initial detection of arrhythmias.

External device 24 may also allow a user to program A-V and/or V-Vintervals for CRT. For example, external device 24A may allow a user toselect an A-V interval, and program ICD 10A to trigger PD 12A to deliverventricular pacing pulse at certain time after a detected P-wave basedon the selected A-V interval, or program PD 12A to deliver ventricularpacing pulse at a certain time after a trigger signal from ICD 10A basedon the selected A-V interval. External device 24 may also, oralternatively, be configured to adjust parameters defining communicationsuch as the duration of windows, the rate of windows, rate ofsynchronization signals, allowable lapses in communication before one ormore devices attempt to re-establish communication, and other suchparameters. External device 24 may also allow a user to programparameters used by processing circuitry, e.g., of ICD 10A, PD 12A,and/or external device 24, to identify features of a cardiac contractionwithin a motion signal, and determine whether the cardiac contraction isa fusion beat or another type of beat, such as an intrinsic beat orfully-paced beat, based on the one or more features, according to thetechniques of this disclosure.

External device 24 may communicate with PD 12A and/or ICD system 4A viawireless communication using any techniques known in the art. Examplesof communication techniques may include, for example, proprietary andnon-proprietary radiofrequency (RF) telemetry, inductive telemetry,acoustics, and tissue conduction communication (TCC), but othertechniques are also contemplated. During TCC, current is driven throughthe tissue between two or more electrodes of a transmitting device. Theelectrical signal spreads and can be detected at a distance by measuringthe voltage generated between two electrodes of a receiving device.

In some examples, PD 12A and ICD 10A may engage in communication tofacilitate the appropriate detection of arrhythmias and/or appropriatedelivery of pacing therapy. The communication may include one-waycommunication in which one device is configured to transmitcommunication messages and the other device is configured to receivethose messages according to the respective schedule. The communicationmay instead include two-way communication in which each device isconfigured to transmit and receive communication messages. Both of PD12A and ICD 10A may be configured to toggle between one-waycommunication modes and two-way communication modes based on the therapythat patient 14 may need. The communication may be via TCC or othercommunication signals, e.g., RF communication signals.

Although PD 12A may at least partially determine if PD 12A delivers CRTor another therapy to patient 14, PD 12A in some examples may performone or more functions in response to receiving a request from ICD 10Aand without any further analysis by PD 12A. In this manner, ICD 10A mayact as a master device and PD 12A may act as a slave device.

Although ICD 10A and PD 12A may perform coordinated communication toprovide CRT and other pacing therapies, these medical devices mayprovide other therapies to patient 14 using transmission andcommunication schedules described herein. For example, ICD 10A may be asubcutaneous, substernal, or transvenous device (although discussed as asubsternal device with respect to FIG. 1A) that detects the atrialdepolarization (i.e., P-wave) and transmits the control signal telling aleadless pacer in the left ventricle (LV) (e.g., PD 12A) when to delivera pacing signal to add CRT to the functionality of ICD 10A. In anotherexample, any device may be implanted subcutaneously in the torso ofpatient 14 to detect the atrial depolarization (P-wave) and transmit acontrol signal to PD 12A in the left ventricle, or PDs in bothventricles, to deliver CRT or other forms of ventricular pacing to heart16A timed to the occurrence of the atrial depolarization.

In another example, two PDs (e.g., including PD 12A illustrated in FIG.1A) may be in communication during ventricular pacing with atrialsensing (VDD) with one PD in the right ventricle to detect the P-wave,deliver pacing signals to and sense activity from the right ventricle,and send a TCC or other signal to PD 12A in the left ventricle todeliver a pacing signal to implement atrial synchronous bi-ventricular(bi-V) pacing. This pacing mode may avoid pacing on a T-wave following aPVC because the PD implanted in the right ventricle may provide sensingand provides backup ventricular pacing and sensing with ventricularevent inhibition (VVI) pacing therapy if the TCC signals between thedevices are lost.

FIG. 2 is a conceptual diagram illustrating an example front view ofpatient 14 implanted with another example medical device system 8B thatincludes an insertable cardiac monitoring (ICM) device 10B that isinserted subcutaneously or substernally in the patient, and PD 12Bimplanted either epicardially or within a cardiac chamber of patient 14,in accordance with one or more aspects of this disclosure. Componentsillustrated in FIG. 2 with like numbers of those of FIGS. 1A-1C may besimilarly configured and may provide similar functionality to thesimilarly-numbered components illustrated in FIGS. 1A-1C. Medical devicesystem 8B of FIG. 2 may leverage cardiac signal sensing capabilities ofICM 10B or PD 12B to perform one or more of the techniques describedherein with respect to optimizing the operation of PD 12B and thecollecting, measuring, and storing various forms of diagnostic data,including generating any corresponding reports or alerts. In certaincases, ICM 10B or PD 12B may directly analyze collected diagnostic dataand generate any corresponding reports or alerts. In some cases,however, ICM 10B or PD 12B may send diagnostic data to external device24. In some examples, ICM 10B may take the form of a Reveal LINQ™ ICM,available from Medtronic plc, of Dublin, Ireland.

Medical device system 8A of FIGS. 1A-1C and medical device system 8B ofFIG. 2 may each be configured to perform the CRT adaptation and thetabulating and reporting of CRT diagnostic data techniques of thisdisclosure. As such, the CRT adaptation and reporting techniques of thisdisclosure are described hereinafter as being performed generically by“medical device system 8,” “implantable medical device (IMD) 10,” whichmay include as examples ICD 10A and ICM 10B, and/or “PD 12,” although itwill be appreciated that the described techniques may be performed bythe respective corresponding systems/devices illustrated in FIGS. 1A-1Cor FIG. 2. In accordance with various aspects of this disclosure,medical device system 8 and/or components thereof may be configured todetect activity of heart 16, and deliver pacing therapy in a timedrelationship to such activity based on a stored interval, e.g., an A-Vor V-V interval. As examples, medical device system 8 may store intervalinformation adapted according to the techniques described in thisdisclosure, or other programming or diagnostic data, to one or morememory devices that are included in the components illustrated in FIGS.1A-1C and 2, and/or to memory device(s) that are otherwisecommunicatively coupled to one or more of the illustrated components ofmedical device system 8.

As described above, IMDs 10 may, in various examples, representdifferent types of cardiac monitoring (and in some cases therapy)devices that can be implanted substernally, subcutaneously, or elsewherein the body of patient 14. In any of these implementations, IMD 10includes interface hardware and sensing circuitry that senses a cardiacsignal that varies as a function of a cardiac cycle of heart 16. Forinstance, the sensing circuitry of IMD 10 may detect the timing ofcardiac depolarization and/or mechanical cardiac contraction events,based on the cardiac signal that varies as a function of the cardiaccycle.

Based on signals from such sensing circuitry, or based on otherinformation (e.g., indicating delivery of pacing pulse), processingcircuitry of IMD 10 may detect the occurrence of atrial and/orventricular depolarizations (paced or intrinsic), or other atrial orventricular events or fiducials from which to set the beginning or endof an A-V or V-V timing interval. In response to detection of thedepolarizations, the processing circuitry may trigger PD 12 to deliverventricular pacing pulses for CRT. The delivery of ventricular pacingpulses for CRT may be timed based on a stored A-V or V-V interval andthe time that the detected atrial or ventricular depolarizationoccurred.

According to the techniques of this disclosure, a motion sensor withinor coupled to PD 12, e.g., a three-axis accelerometer, may generate asignal that indicates mechanical activity of the heart, includingcontractions, during the delivery of CRT. According to the techniques ofthis disclosure, processing circuitry, e.g., of PD 12, IMD 10, externaldevice 24, and/or any device described herein, may identify one or morefeatures of cardiac contractions with the motion signal, and determinewhether the cardiac contraction is one of a fusion beat, intrinsic beat,or fully-paced beat based on the one or more features. Fusion beats aredesired, and the processing circuitry may adapt the CRT pacing controlparameter values, e.g., the A-V, V-V or other timing intervals, toachieve fusion beats based on detection of one or more of the othertypes of beats. The processing circuitry may also generate diagnosticinformation based on the numbers of beats of the different types (e.g.,fusion vs. non-fusion), which may be reported to a user. The diagnosticinformation may include values of one or more metrics indicating anamount, e.g., a percentage, of beats that are fusion beats or otherwiseeffectively captured the heart.

FIG. 3 is a conceptual drawing illustrating an example configuration ofICM 10B illustrated in FIG. 2. In the example shown in FIG. 3, ICM 10Bmay be embodied as a monitoring device having housing 32, proximalelectrode 34 and distal electrode 36. Housing 32 may further includefirst major surface 38, second major surface 40, proximal end 42, anddistal end 44. Housing 32 encloses electronic circuitry located insidethe ICM 10B and protects the circuitry contained therein from bodyfluids. Electrical feedthroughs provide electrical connection ofelectrodes 34 and 36.

In the example shown in FIG. 3, ICM 10B is defined by a length L, awidth W and thickness or depth D and is in the form of an elongatedrectangular prism wherein the length L is much larger than the width W,which in turn is larger than the depth D. In one example, the geometryof the ICM 10B—a width W greater than the depth D—is selected to allowICM 10B to be inserted under the skin of the patient using a minimallyinvasive procedure and to remain in the desired orientation duringinsertion. For example, the device shown in FIG. 3 includes radialasymmetries (notably, the rectangular shape) along the longitudinal axisthat maintains the device in the proper orientation following insertion.For example, in one example the spacing between proximal electrode 34and distal electrode 36 may range from thirty millimeters (mm) tofifty-five mm, thirty-five mm to fifty-five mm, and from forty mm tofifty-five mm and may be any range or individual spacing fromtwenty-five mm to sixty mm.

In addition, ICM 10B may have a length L that ranges from thirty mm toabout seventy mm. In other examples, the length L may range from fortymm to sixty mm, forty-five mm to sixty mm and may be any length or rangeof lengths between about thirty mm and about seventy mm. In addition,the width W of major surface 38 may range from three mm to ten mm andmay be any single or range of widths between three mm and ten mm. Thethickness of depth D of ICM 10B may range from two mm to nine mm. Inother examples, the depth D of ICM 10B may range from two mm to five mmand may be any single or range of depths from two mm to nine mm.

Furthermore, ICM 10B according to an example of the present disclosureis has a geometry and size designed for ease of implant and patientcomfort. Examples of ICM 10B described in this disclosure may have avolume of three cubic centimeters (cm) or less, one-and-a-half cubic cmor less or any volume between three and one-and-a-half cubiccentimeters. In addition, in the example shown in FIG. 3, proximal end42 and distal end 44 are rounded to reduce discomfort and irritation tosurrounding tissue once inserted under the skin of the patient. In someexamples, ICM 10B, including instrument and method for inserting ICM 10Bis configured as described, for example, in U.S. Patent Publication No.2014/0276928, which is entitled, “SUBCUTANEOUS DELIVERY TOOL,” andpublished on Sep. 18, 2014. U.S. Patent Publication No. 2014/0276928 toVanderpool et al. is incorporated herein by reference in its entirety.In some examples, ICM 10B is configured as described, for example, inU.S. Patent Publication No. 2016/0310031, which is entitled, “METHOD ANDAPPARATUS FOR DETERMINING A PREMATURE VENTRICULAR CONTRACTION IN AMEDICAL MONITORING DEVICE,” and published on Oct. 27, 2016. U.S. PatentPublication No. 2016/0310031 to Sarkar is incorporated herein byreference in its entirety.

In the example shown in FIG. 3, once inserted within the patient, thefirst major surface 38 faces outward, toward the skin of the patientwhile the second major surface 40 is located opposite the first majorsurface 38. Consequently, the first and second major surfaces may facein directions along a sagittal axis of patient 14A (e.g., see FIG. 2),and this orientation may be consistently achieved upon implantation dueto the dimensions of ICM 10B. Additionally, an accelerometer, or axis ofan accelerometer, may be oriented along the sagittal axis.

Proximal electrode 34 and distal electrode 36 are used to sense cardiacsignals, e.g., cardiac EGM signals, intra-thoracically orextra-thoracically, which may be sub-muscularly or subcutaneously. EGMsignals may be stored in a memory of the ICM 10B, and EGM data may betransmitted via integrated antenna 52 to another medical device, whichmay be another implantable device or an external device, such asexternal device 14A. In some example, electrodes 34 and 36 mayadditionally or alternatively be used for sensing any bio-potentialsignal of interest, which may be, for example, any EGM,electroencephalogram (EEG), electromyogram (EMG), or a nerve signal,from any implanted location.

In the example shown in FIG. 3, proximal electrode 34 is close to theproximal end 42, and distal electrode 36 is close to distal end 44. Inthis example, distal electrode 36 is not limited to a flattened, outwardfacing surface. Distal electrode 36 may extend from first major surface38 around rounded edges 46 and/or end surface 48 and onto the secondmajor surface 40 so that the electrode 36 has a three-dimensional curvedconfiguration. In the example shown in FIG. 3, proximal electrode 34 islocated on first major surface 38 and is substantially flat, outwardfacing. However, in other examples, proximal electrode 34 may utilizethe three-dimensional curved configuration illustrated with respect todistal electrode 36 in FIG. 3, providing a three-dimensional proximalelectrode. In other examples still, distal electrode 36 may utilize asubstantially flat, outward facing electrode located on first majorsurface 38 like that shown in FIG. 3 with respect to proximal electrode34.

The various electrode configurations allow for configurations in whichproximal electrode 34 and distal electrode 36 are located on both firstmajor surface 38 and second major surface 40. In other configurations,such as the configuration shown in FIG. 3, only one of proximalelectrode 34 or distal electrode 36 is located on both major surfaces 38and 40. In still other configurations, both proximal electrode 34 anddistal electrode 36 are located on one of the first major surface 38 orthe second major surface 40 (i.e., proximal electrode 34 may be locatedon first major surface 38 while distal electrode 36 may be located onsecond major surface 40). In another example, ICM 10B may includeelectrodes on both major surface 38 and 40 at or near the proximal anddistal ends of the device, such that a total of four electrodes areincluded on ICM 10B. Electrodes 34 and 36 may be formed of a pluralityof different types of biocompatible conductive material, e.g., stainlesssteel, titanium, platinum, iridium, or alloys thereof, and may utilizeone or more coatings such as titanium nitride or fractal titaniumnitride.

In the example shown in FIG. 3, proximal end 42 includes a headerassembly 50 that includes one or more of proximal electrode 34,integrated antenna 52, anti-migration projections 54, and/or suture hole56. Integrated antenna 52 is located on the same major surface (i.e.,first major surface 38) as proximal electrode 34 and is also included aspart of header assembly 50. Integrated antenna 52 allows ICM 10B totransmit and/or receive data. In other examples, integrated antenna 52may be formed on the opposite major surface as proximal electrode 34, ormay be incorporated within the housing 32 of ICM 10B. In the exampleshown in FIG. 3, anti-migration projections 54 are located adjacent tointegrated antenna 52 and protrude away from first major surface 38 toprevent longitudinal movement of the device. In the example shown inFIG. 3 anti-migration projections 54 includes a plurality (e.g., nine)small bumps or protrusions extending away from first major surface 38.

As discussed above, in other examples, anti-migration projections 54 maybe located on the opposite major surface as proximal electrode 34 and/orintegrated antenna 52. In addition, in the example shown in FIG. 3header assembly 50 includes suture hole 56, which provides another meansof securing ICM 10B to the patient to prevent movement following insert.In the example shown, suture hole 56 is located adjacent to proximalelectrode 34. In one example, header assembly 50 is a molded headerassembly made from a polymeric or plastic material, which may beintegrated or separable from the main portion of ICM 10B.

FIG. 4 is a conceptual drawing illustrating an example PD 12, which maycorrespond to either or both of PD 12A of FIG. 1A or PD 12B of FIG. 2.As shown in FIG. 4, PD 12 includes case 50, cap 58, electrode 60,electrode 52, fixation mechanisms 62, flange 54, and opening 56.Together, case 50 and cap 58 may be considered the housing of PD 12. Inthis manner, case 50 and cap 58 may enclose and protect the variouselectrical components within PD 12. Case 50 may enclose substantiallyall the electrical components, and cap 58 may seal case 50 and createthe hermetically sealed housing of PD 12. Although PD 12 is generallydescribed as including one or more electrodes, PD 12 may typicallyinclude at least two electrodes (e.g., electrodes 52 and 60) to deliveran electrical signal (e.g., therapy such as CRT) and/or provide at leastone sensing vector. Electrodes 52 and 60 are carried on the housingcreated by case 50 and cap 58. In this manner, electrodes 52 and 60 maybe considered leadless electrodes.

In the example of FIG. 4, electrode 60 is disposed on the exteriorsurface of cap 58. Electrode 60 may be a circular electrode positionedto contact cardiac tissue upon implantation. Electrode 52 may be a ringor cylindrical electrode disposed on the exterior surface of case 50.Both case 50 and cap 58 may be electrically insulating. Electrode 60 maybe used as a cathode, and electrode 52 may be used as an anode, or viceversa, for delivering CRT or other appropriate cardiac therapy(bradycardia pacing, ATP, post-shock pacing, etc.). However, electrodes52 and 60 may be used in any stimulation configuration. In addition,electrodes 52 and 60 may be used to detect intrinsic electrical signalsfrom cardiac muscle. In other examples, PD 12 may include three or moreelectrodes, where each electrode may deliver therapy and/or detectintrinsic signals. CRT and other pacing delivered by PD 12 may be“painless” to patient 14 or even undetectable by patient 14 since theelectrical stimulation occurs very close to or at cardiac muscle and atrelatively low energy levels compared with alternative devices.

Fixation mechanisms 62 may attach PD 12 to cardiac tissue. Fixationmechanisms 62 may be active fixation tines, screws, clamps, adhesivemembers, or any other types of attaching a device to tissue. As shown inthe example of FIG. 4, fixation mechanisms 62 may be constructed of amemory material that retains a preformed shape. During implantation,fixation mechanisms 62 may be flexed forward to pierce tissue andallowed to flex back towards case 50. In this manner, fixationmechanisms 62 may be embedded within the target tissue.

Flange 54 may be provided on one end of case 50 to enable tethering orextraction of PD 12. For example, a suture or other device may beinserted around flange 54 and/or through opening 56 and attached totissue. In this manner, flange 54 may provide a secondary attachmentstructure to tether or retain PD 12 within heart 16 if fixationmechanisms 62 fail. Flange 54 and/or opening 56 may also be used toextract PD 12 once the PD needs to be explanted (or removed) frompatient 14 if such action is deemed necessary.

In another example, PD 12 may be configured to be implanted external toheart 16, e.g., near or attached to the epicardium of heart 16. Anelectrode carried by the housing of the fusion pacing PD 12 may beplaced in contact with the epicardium and/or one or more electrodesplaced in contact with the epicardium at locations sufficient to providetherapy (e.g., on external surfaces of the left and/or rightventricles). In any example, IMD 10 may communicate with one or moreleadless or leaded devices implanted internal or external to heart 16.

FIG. 5 is a block diagram of an example configuration of an IMD 10 thatis configured according to one or more aspects of this disclosure. IMD10 of FIG. 5 may, in various use case scenarios, represent an example ofICD 10A of FIGS. 1A-1C or ICM 10B of FIG. 2. IMD 10 includes two or moreelectrodes 71A-N (collectively “electrodes 71”), which may correspond todefibrillation electrodes 20 (FIGS. 1A-C), sensing electrodes 22 FIGS.1A-C), one or more housing electrodes of ICD 10A (FIGS. 1A-C), orelectrodes 34 and 36 (FIG. 3). IMD 10 may include processing circuitry70 for controlling sensing circuitry 76, communication circuitry 78,(optionally) switching circuitry 72, memory 82, and (optionally) therapygeneration circuitry 80. The optional nature of switching circuitry 72and therapy generation circuitry 80 is shown using dashed-line bordersto indicate the optional aspect, in FIG. 5. As one example, therapygeneration circuitry 80 is indicated as optional because someembodiments of an IMD configured as an ICM do not deliver therapy.Switching circuitry 72 may include one or more switches, such asmetal-oxide-semiconductor field-effect transistors (MOSFETs) or bipolartransistors. Processing circuitry 70 may control switching circuitry 72to connect selected groupings of electrodes 71 to sensing circuitry 76to sense one or more physiological electrical signals.

Sensing circuitry 76 is configured to receive cardiac electrical signalsfrom selected combinations of two or more electrodes 71, and sensecardiac events attendant to depolarization and repolarization of cardiactissue. Sensing circuitry 76 may include one or more sensing channels,each of which may be selectively coupled to respective combinations ofelectrodes 71 to detect electrical activity of a particular chamber ofheart 16, e.g., one or more atrial and/or ventricular sensing channels.Each sensing channel may be configured to amplify, filter and rectifythe cardiac electrical signal received from selected electrodes coupledto the respective sensing channel to detect cardiac events, e.g., P-waveand R-waves. For example, each sensing channel may include one or morefilters and amplifiers for filtering and amplifying a signal receivedfrom a selected pair of electrodes. The resulting cardiac electricalsignal may be passed to cardiac event detection circuitry that detects acardiac event when the cardiac electrical signal crosses a sensingthreshold. The cardiac event detection circuitry may include arectifier, filter and/or amplifier, a sense amplifier, comparator,and/or analog-to-digital converter. Sensing circuitry 76 may output anindication to processing circuitry 70 in response to sensing a cardiacevent in a chamber of interest, e.g., a P-wave or R-wave. In thismanner, processing circuitry 70 may receive detected cardiac eventsignals corresponding to the occurrence of detected P-waves and R-waves.Indications of detected R-waves may be used by processing circuitry 70for detecting ventricular arrhythmia episodes, and indications ofdetected P-waves may be used by processing circuitry 70 for detectingatrial arrhythmia episodes. Sensing circuitry 76 may also pass one ormore digitized EGM signals to processing circuitry 70 for analysis,e.g., for use in cardiac rhythm discrimination and for morphologicalanalysis.

Communication circuitry 78 may include circuitry for generating andmodulating, and in some cases receiving and demodulating, continuousand/or pulsatile communication waveforms. Communication circuitry 78 maybe configured to transmit and/or receive one or both of RF signals viaan antenna (not shown) or TCC signals via electrodes 71. Although notshown in FIG. 5, communication circuitry 78 may be coupled to a selectedtwo or more electrodes 71 via switching circuitry 72 for TCC.

In some examples, processing circuitry 70 may control switchingcircuitry 72 to connect electrodes 71 to therapy generation circuitry 80to deliver a therapy pulse, such as a pacing, cardioversion, ordefibrillation pulse to the heart. Therapy generation circuitry 80 iselectrically coupleable to electrodes 71, and is configured to generateand deliver electrical therapy to heart 16 via selected combinations ofelectrodes 71. Therapy generation circuitry 80 may include chargingcircuitry, and one or more charge storage devices, such as one or morehigh voltage capacitors and/or one or more low voltage capacitors.Switching circuitry 72 may control when the capacitor(s) are dischargedto selected combinations of electrodes 71. Therapy generation circuitry80 and/or processing circuitry 70 may control the frequency, amplitude,and other characteristics of the therapy pulses. Therapy generationcircuitry 80 may deliver the therapy pulses to electrodes 71 whenswitching circuitry 72 connects therapy generation circuitry 80 toelectrodes 71.

Processing circuitry 70 may control switching circuitry 72 by sendingcontrol signals to the control terminals of one or more switches ofswitching circuitry 72. The control signals may control whether theswitches of switching circuitry 72 conduct electricity between the loadterminals of the switches. If switching circuitry 72 includes MOSFETswitches, the control terminals may include gate terminals, and the loadterminals may include drain terminals and source terminals.

Processing circuitry 70 may include various types of hardware, includingone or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalentintegrated or discrete logic circuitry, as well as any combinations ofsuch components. The term “processing circuitry” may generally refer toany of the foregoing logic circuitry, alone or in combination with otherlogic circuitry, or any other equivalent circuitry. Processing circuitry70 represents hardware that can be configured to implement firmwareand/or software that sets forth one or more of the algorithms describedherein. Memory 82 includes computer-readable instructions that, whenexecuted by processing circuitry 70, cause IMD 10 and processingcircuitry 70 to perform various functions attributed to IMD 10 andprocessing circuitry 70 herein. Memory 82 may include any volatile,non-volatile, magnetic, optical, or electrical media, such as arandom-access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other digital media.

In some examples, processing circuitry 70 receives indications of theoccurrence of P-waves or other atrial events (or R-wave or otherventricular events) from sensing circuitry 76, or identifies theoccurrence of P-waves or other atrial events by processing of afar-field EGM signal received from sensing circuitry using any of avariety of techniques known in the art. In response to the atrial orventricular event, processing circuitry 70 may control communicationcircuitry 78 to transmit a signal to intracardiac PD 12. The signalcauses intracardiac PD 12 to deliver a ventricular pacing pulse for CRT,absent an intrinsic ventricular depolarization prior to expiration of atiming interval, such as an A-V or V-V interval. As described above, oneor both of IMD 10 and intracardiac PD 12 may store adjustable timingintervals that control the delivery of CRT based on an A-V interval orV-V interval. IMD 10 may store such intervals in memory 82.

In some examples, processing circuitry 70 of IMD 10 may receive a motionsignal from a sensor of intracardiac PD 12 that indicates mechanicalactivity of the heart, e.g., motion of the intracardiac PD and itssensor within the heart during contraction. Processing circuitry 70 may,according to the techniques described herein, identify one or morefeatures of a cardiac contraction within the signal, determine whetherthe cardiac contraction is a fusion beat based on the one or morefeatures, and control a timing interval or other control parameter valuefor delivery of ventricular pacing for CRT by PD 12 based on thedetermination. Processing circuitry 70 may control a timing intervalstored in memory 82 and, for example, used by the processing circuitryto determine when to transmit a signal to PD 12 or when to instruct PDto deliver a ventricular pacing pulse. Processing circuitry 70 maytransmit a signal to intracardiac PD 12 via communication circuitry 78to adjust a timing interval used by the intracardiac PD. Processingcircuitry 70 may also determine values of one more metrics that indicatethe effectiveness of CRT based on the determination of whether thecontractions during CRT are fusion beats according to the techniquesdescribed herein.

Existing techniques for illustrating the effectiveness of CRT includedetermining the percentage of beats that are paced. Such techniqueshowever, do not account for whether the paced beats successfullycaptured the heart. IMDs implementing the EffectivCRT™ algorithm,available from Medtronic plc of Dublin, Ireland, determine thepercentage or other amount of CRT pacing that was actually effective incapturing the heart, e.g., based on the morphology of resulting cardiacsignal in a particular observation vector, such as from aleft-ventricular pacing cathode to a right-ventricular coil electrode ina system including such electrodes. An example of EffectivCRT™ isdescribed in U.S. Pat. No. 8,750,998 to Ghosh et al., which is entitled,“EFFECTIVE CAPTURE TEST,” and issued on Jun. 10, 2014. U.S. Pat. No.8,750,998 to Ghosh et al. is incorporated herein by reference in itsentirety. Another example of use of the EffectivCRT™ algorithm tomaintain effective CRT during atrial fibrillation is described in U.S.Patent Publication No. 2014/0277245 to Lu et al., which is entitled,“MODULATE PACING RATE TO INCREASE THE PERCENTAGE OF EFFECTIVEVENTRICULAR CAPTURE DURING ATRIAL FIBRILLATION,” and published on Sep.18, 2014. U.S. Patent Publication No. 2014/0277245 to Lu et al. isincorporated herein by reference in its entirety.

FIG. 6 is a functional block diagram illustrating an exampleconfiguration of PD 12, which may correspond to PD 12A of FIGS. 1A-1C orPD 12B of FIG. 2. In the illustrated example, PD 12 includes processingcircuitry 90, memory 92, therapy generation circuitry 96, sensingcircuitry 98, motion sensor 100, and communication circuitry 94. Memory92 includes computer-readable instructions that, when executed byprocessing circuitry 90, cause PD 12 and processing circuitry 90 toperform various functions attributed to PD 12 and processing circuitry90 herein (e.g., analyzing a motion signal from sensor 100 thatindicates mechanical activity of the heart, e.g., motion of theintracardiac PD and sensor 100 within the heart during contraction,identifying one or more features of a cardiac contraction within thesignal, determining whether the cardiac contraction is a fusion beatbased on the one or more features, controlling a timing interval fordelivery of ventricular pacing for CRT by therapy generation circuitry96 based on the determination, and determining values of one moremetrics that indicate the effectiveness of CRT based on thedetermination). Memory 92 may include any volatile, non-volatile,magnetic, optical, or electrical media, such as a random-access memory(RAM), read only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital or analog media.

Processing circuitry 90 may include any one or more of a microprocessor,a controller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA), orequivalent discrete or analog logic circuitry. In some examples,processor 90 may include multiple components, such as any combination ofone or more microprocessors, one or more controllers, one or more DSPs,one or more ASICs, or one or more FPGAs, as well as other discrete orintegrated logic circuitry. The functions attributed to processingcircuitry 90 herein may be embodied as software, firmware, hardware orany combination thereof.

Processing circuitry 90 controls therapy generation circuitry 96 todeliver stimulation therapy to heart 16 according to therapy parameters,which may be stored in memory 92. For example, processing circuitry 90may control therapy generation circuitry 96 to deliver electrical pulseswith the amplitudes, pulse widths, frequency, or electrode polaritiesspecified by the therapy parameters. In this manner, therapy generationcircuitry 96 may deliver pacing pulses (e.g., fusion pacing for CRT) toheart 16 via electrodes 52 and 60. Although PD 12 may only include twoelectrodes, e.g., electrodes 52 and 60, PD 12 may utilize three or moreelectrodes in other examples. PD 12 may use any combination ofelectrodes to deliver therapy and/or detect electrical signals frompatient 14.

Therapy generation circuitry 96 is electrically coupled to electrodes 52and 60 carried on the housing of PD 12. In the illustrated example,therapy generation circuitry 96 is configured to generate and deliverelectrical stimulation therapy to heart 16. For example, therapygeneration circuitry 96 may deliver pulses to a portion of cardiacmuscle within heart 16 via electrodes 52 and 60. In some examples,therapy generation circuitry 96 may deliver pacing stimulation in theform of electrical pulses. In other examples, signal generator maydeliver one or more of these types of stimulation in the form of othersignals, such as sine waves, square waves, or other substantiallycontinuous time signals. Therapy generation circuitry 96 may includecharging circuitry, and one or more charge storage devices, such as oneor more capacitors. Switching circuitry (not shown) may control when thecapacitor(s) are discharged to electrodes 52 and 60.

Sensing circuitry 98 monitors signals from at least one of electrodes 52and 60 to monitor electrical activity of heart 16, impedance, or anotherelectrical phenomenon. Sensing may be done to determine heart rates orheart rate variability, or to detect ventricular dyssynchrony,arrhythmias (e.g., tachyarrhythmias) or other electrical signals.Sensing circuitry 98 may include switching circuitry to select theelectrode polarity used to sense the heart activity. In examples withmore than two electrodes, processing circuitry 90 may select theelectrodes that function as sense electrodes, i.e., select the sensingconfiguration, via the switching circuitry within sensing circuitry 98.Sensing circuitry 98 may include one or more detection channels, each ofwhich may be coupled to a selected electrode configuration for detectionof cardiac signals via that electrode configuration. Some detectionchannels may be configured to detect cardiac events, such as R-waves,and provide indications of the occurrences of such events to processingcircuitry 90, e.g., as described in U.S. Pat. No. 5,117,824 to Keimel etal., which issued on Jun. 2, 1992 and is entitled, “APPARATUS FORMONITORING ELECTRICAL PHYSIOLOGIC SIGNALS,” and is incorporated hereinby reference in its entirety. Processing circuitry 90 may control thefunctionality of sensing circuitry 98 by providing signals via adata/address bus.

In addition to detecting and identifying specific types of cardiacrhythms (types of cardiac events), sensing circuitry 98 may also samplethe detected intrinsic signals to generate an electrogram or othertime-based indication of cardiac events. Processing circuitry 90 mayalso be able to coordinate the delivery of pacing pulses from differentPDs, e.g., implanted in different chambers of heart 16, such as an PDimplanted in the other ventricle. For example, processing circuitry 90may identify delivered pulses from other PDs via sensing circuitry 98and updating pulse timing. In other examples, PDs may communicate witheach other via communication circuitry 94 and/or instructions over acarrier wave (such as a stimulation waveform).

Memory 92 may be configured to store a variety of operationalparameters, therapy parameters, sensed and detected data, and any otherinformation related to the therapy and treatment of patient 14. In theexample of FIG. 6, memory 92 may store sensed EGMs, signals receivedfrom motion sensor 100, communications from IMD 10, therapy parametervalues, such as timing intervals that control the timing of CRT pacingpulses or other CRT control parameter values, information indicatingwhether cardiac contractions during CRT were fusion or other beats, andvalues of one or more metrics that indicate CRT effectiveness. In someexamples, memory 92 may act as a temporary buffer for storing data untilit can be uploaded to IMD 10, another implanted device, or externaldevice 24.

Motion sensor 100 may be contained within the housing of PD 12 andinclude one or more accelerometers, gyroscopes, electrical or magneticfield sensors, or other devices capable of detecting motion and/orposition of PD 12. For example, motion sensor 100 may include a 3-axisaccelerometer (three-dimensional accelerometer) that is configured todetect accelerations in any direction in space. Specifically, the 3-axisaccelerometer may be used to detect PD 12 motion that may be indicativeof cardiac events and/or noise. For example, processing circuitry 90 maymonitor the accelerations from motion sensor 100 to confirm or detectarrhythmias. Since PD 12 may move with a chamber wall of heart 16, thedetected changes in acceleration may also be indicative of contractions.Therefore, PD 12 may be configured to, based on the signal from sensor100, identify heart rates and confirm ventricular dyssynchrony sensedvia sensing circuitry 98.

When processing circuitry 90 controls therapy generation circuitry 96 todeliver ventricular pacing pulses for CRT, processing circuitry 90 mayalso control motion sensor(s) 100 to generate a signal that varies withthe cardiac contraction. In some examples, motion sensor(s) 100 maygenerate the signal substantially continuously. For each cardiac cycleduring which a ventricular pacing pulses is delivered, processingcircuitry 90 may identify one or more features of the cardiaccontraction within the signal. Processing circuitry 90 may determinewhether the contraction is a fusion beat or other type of beat, e.g.,intrinsic or fully-paced, based on the one or more features.

The one or more features of the cardiac contraction may comprise, asexamples, one or more of a slope of the motion signal during the cardiaccontraction, an amplitude of the motion signal during the contraction, amaximum of the signal during the cardiac contraction, a minimum of thesignal during the cardiac contraction, a ratio of the maximum and theminimum during the cardiac contraction, or a polarity of the signalduring the cardiac contraction. As described in greater detail below,such as with respect to FIGS. 11A and 11B, the one or more features ofthe cardiac contraction within the signal may comprise one or more of anamount or a direction of motion relative to a point of origin during thecontraction. In some examples, the amount of motion is in at least onedirection other than the primary axis of motion during the cardiaccontraction, e.g., an amount of motion in at least one plane orthogonalto the primary axis of motion during the cardiac contraction. In someexamples, the motion of interest may be radial motion in the planeorthogonal to the primary axis of motion during the cardiac contraction.

In examples in which one or more sensor(s) 100 are other than anaccelerometer, the features used to evaluate the cardiac contractionusing the techniques described herein may be the same or different thanthose provided by an accelerometer. For example, a gyroscope may providerotational mechanical information different from that provided byaccelerometers, but may similarly be used to evaluate contractions. Forexample, template signals or values may be determined based on suchsignals and compared to current signals or values.

During left-ventricular contraction of a relatively healthy heart, allleft-ventricular walls shorten synchronously and with similar force.Such a contraction pulls the atrio-ventricular plane and theleft-ventricular apex toward each other. The movement of theatrio-ventricular plane and the left-ventricular apex toward each otherdefines the primary axis of motion during the cardiac contraction.

In the case of a totally synchronous activation of all left-ventricularwalls, there is a total force balance at the apex, and there is minimalor no resulting radial motion component of the apex. In contrast, duringasynchronous contraction, “apical rocking” (e.g., movement of the apexduring systole, first toward the free wall and later toward the septum)or other motion of the apex or other heart tissue in a directionorthogonal to the primary axis can be observed. Several studies haveshown that observation of apical rocking is predictive of subsequent CRTresponse. Apical rocking has been measured by echocardiography. Forexample, from a “4-chamber view,” the apex moves 5 mm towards thelateral or free wall of the left ventricle during systole. The 4-chamberview is a 2-dimensional standardized echocardiographic visualization ofthe heart. The 4-chamber view visualizes the heart from the apex of the(left) cardiac chamber. The visualization plane through the heart isdirected from the apex of the heart in such a way, that all 4 cardiacchambers (right atrium, left atrium, right ventricle and left ventricle)are visualized at the same time.

Motion sensor(s) 100 being located at or near the apex, e.g., because PD12 is implanted at that location, may be configured to detect apicalrocking. However, motion sensor(s) 100 being implanted at otherlocations, e.g., on the ventricular free wall or septum, may also detectmotion in a direction other than the primary axis of motion during thecardiac contraction. Further, motion sensor(s) 100 may detect motion ordisplacement in any radial direction away from the origin point of thecardiac cycle.

In some examples, the one or more the one or more features of thecardiac contraction within the signal comprise one or more valuesresulting from comparison of the signal during the cardiac contractionto one or more templates. Such values may include differences betweenthe signal the template, which may be compared to thresholds todetermine whether the signal “matches” the template, e.g., issufficiently similar to the template. The one or more templates may bestored in memory 92 (or another memory of system 8). The one or moretemplates may be generated by processing circuitry 90 or otherprocessing circuitry based on the motion signal during one or moreprevious beats of known classification (such as by averaging the signalfor a plurality of known beats of a given classification), either of theparticular patient, or from a population of one or more similarpatients. The one or more templates may include one or more of anintrinsic beat template, a fully-paced beat template, or a fusion beattemplate, and processing circuitry may characterize a given contractionas the one of a fusion beat, intrinsic beat, or fully-paced beat whosetemplate the signal best matches.

Further, the templates need not take the form of one or moresubstantially continuous-time signals, representing a number of values,from one or more sensors during a contraction (or portion of such asignal). Rather, a template may take the form of a template value forany one or more of the contraction features disclosed herein. Thetemplate value may, but need not necessarily be, determined based on atemplate signal or otherwise determined from motion signals collectedduring one or more previous beats of known classification (such as byaveraging the signal for a plurality of known beats of a givenclassification), either of the particular patient, or from a populationof one or more similar patients. Template values may include, asexamples, values of: slope of the motion signal during the cardiaccontraction, an amplitude of the motion signal during the contraction, amaximum of the signal during the cardiac contraction, a minimum of thesignal during the cardiac contraction, a ratio of the maximum and theminimum during the cardiac contraction, a polarity of the signal duringthe cardiac contraction, or an amount of motion in one or moreparticular directions, such as orthogonal to the primary axis of motion,which may include apical rocking. The templates can include templatevalues for multiple contraction features. Although described in thecontext of processing circuitry 90 of intracardiac PD 12, processingcircuitry of any one or more devices described herein may similarly usetemplates to classify contractions.

Communication circuitry 94 includes any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice, such as external device 24 or IMD 10, via TCC or RF signals, asdescribed herein. In some examples, communication circuitry 94 may beconfigured for TCC communication with IMD 10 via electrodes 52 and 60.PD 12 may communicate with external device 24 via IMD 10, orcommunication circuitry 94 may be configured for RF communication withexternal device 24, e.g., via an antenna. In some examples, PD 12 maysignal external device 24 to further communicate with and pass the alertthrough a network such as the Medtronic CareLink® Network developed byMedtronic plc of Dublin, Ireland, or some other network linking patient14 to a clinician. PD 12 may spontaneously transmit information to thenetwork or in response to an interrogation request from a user.

There may be numerous variations to the configuration of PD 12, asdescribed herein. In one example, PD 12 includes a housing configured tobe implanted within heart 16 of patient 14, one or more electrodes(e.g., electrodes 52 and 60) coupled to the housing, fixation mechanism62 configured to attach the housing to tissue of heart 16, sensingcircuitry 98 configured to sense an electrical signal from heart 16 ofpatient 14 via the one or more electrodes, and signal generationcircuitry 96 configured to deliver therapy to heart 16 of patient 14 viathe one or more electrodes. PD 12 may also include processing circuitry90 configured to receive a communication message from IMD 10 requestingPD 12 deliver CRT to heart 16, where IMD 10 is configured to beimplanted exterior to a ribcage of patient 14. Processing circuitry 90may also be configured to determine, based on the sensed electricalsignal, whether to deliver CRT to heart 16, and, in response to thedetermination, command signal generation circuitry 96 to deliver the CRTtherapy. For example, processing circuitry 90 may withhold delivery ofCRT pacing pulse during a particular cardiac cycle if sensing circuitry98 indicates detection of an intrinsic R-wave prior to expiration of atiming interval, e.g., an A-V interval. If processing circuitry 90controls signal generation circuitry 96 to deliver the pacing pulse,processing circuitry 90 or other processing circuitry of one or moreother devices may analyze the signal from motion sensor(s) 100 toclassify the cardiac contraction as described herein.

FIG. 7 is a functional block diagram illustrating an exampleconfiguration of external device 24. As shown in FIG. 7, external device24 may include processing circuitry 110, memory 112, user interface 114,and communication circuitry 116. External device 24 may be a dedicatedhardware device with dedicated software for communication with, e.g.,programming of, PD 12 and/or IMD 10. Alternatively, external device 24may be an off-the-shelf computing device running an application thatenables external device 24 to program and/or otherwise communicate withPD 12 and/or IMD 10.

A user may use external device 24 to configure the operationalparameters of and retrieve data from PD 12 and/or IMD 10. In oneexample, external device 24 may communicate directly to both PD 12 andIMD 10. In other examples, external device 24 may communicate to one ofPD 12 and IMD 10, and that device may relay any instructions orinformation to or from the other device. The clinician may interact withexternal device 24 via user interface 114, which may include display topresent graphical user interface to a user, and a keypad or anothermechanism for receiving input from a user. In addition, the user mayreceive an alert or notification from IMD 10 indicating that a shock hasbeen delivered, any other therapy has been delivered, or any problems orissues related to the treatment of patient 14.

Processing circuitry 110 can take the form one or more microprocessors,DSPs, ASICs, FPGAs, programmable logic circuitry, or the like, and thefunctions attributed to processing circuitry 110 herein may be embodiedas hardware, firmware, software or any combination thereof. Memory 112may store instructions that cause processing circuitry 110 to providethe functionality ascribed to external device 24 herein, and informationused by processing circuitry 110 to provide the functionality ascribedto external device 24 herein. Memory 112 may include any fixed orremovable magnetic, optical, or electrical media, such as RAM, ROM,CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory 112may also include a removable memory portion that may be used to providememory updates or increases in memory capacities. A removable memory mayalso allow patient data to be easily transferred to another computingdevice, or to be removed before external device 24 is used to programtherapy for another patient.

External device 24 may communicate wirelessly with PD 12 and/or IMD 10,such as using RF communication or proximal inductive interaction. Thiswireless communication is possible with communication circuitry 116,which may be coupled to an internal antenna or an external antenna. Anexternal antenna that is coupled to external device 24 may correspond tothe programming head that may be placed over heart 16 or the location ofthe intend implant, as described above with reference to FIG. 1.Communication circuitry 116 may be configured with circuitry likecommunication circuitry 78 of FIG. 5.

Communication circuitry 116 may also be configured to communicate withanother computing device via wireless communication techniques, ordirect communication through a wired connection. Examples of localwireless communication techniques that may be employed to facilitatecommunication between external device 24 and another computing deviceinclude RF communication according to the 802.11 or Bluetoothspecification sets, infrared communication, e.g., according to the IrDAstandard, or other standard or proprietary telemetry protocols. Anadditional computing device in communication with external device 24 maybe a networked device such as a server capable of processing informationretrieved from IMD 10 and/or PD 12.

In some examples, processing circuitry 110 may receive a signal fromsensor 100 of PD 12 via direct or indirect communication with PD 12using communication circuitry 116. using the signal, processingcircuitry 110 may, in whole or in part, perform any of the methodsdescribed herein for adapting and evaluating CRT, including one or moreof classifying beats as fusion or other based on the signal, controllinga timing interval or other control parameter value used by IMD 10 and/orPD 12 for timing the delivery of CRT by PD 12 based on theclassification of the beats, and determining values of one or moremetrics indicating an amount of beats that were fusion beats and, thus,the effectiveness of CRT. In some examples, processing circuitry 110 mayreceive values for features of contractions rather than the motionsignal, or classifications of beats rather than the feature values, andperform some portions of methods described herein using the receivedfeatures or classification information.

FIG. 8 is a block diagram illustrating a system 140 that includes anexternal device 142, such as a server, and one or more computing devices144A-144N that are coupled to IMD 10, PD 12, and external device 24 viaa network 150, according to one example. In this example, IMD 10 usescommunication circuitry to communicate with external device 24 via afirst wireless connection and communicates with an access point 152 viaa second wireless connection. PD 12 uses communication circuitry tocommunicate with external device 24 via a first wireless connection andcommunicates with an access point 152 via a second wireless connection.IMD 10 and PD 12 communicate with each other via a shared third wirelessconnection. In the example of FIG. 8, access point 152, external device24, external device 142, and computing devices 144A-144N areinterconnected, and able to communicate with each other, through network150. In some cases, one or more of access point 152, external device 24,external device 142, and computing devices 144A-144N may be coupled tonetwork 150 through one or more wireless connections. IMD 10, PD 12,external device 24, external device 152, and computing devices 144A-144Nmay each comprise one or more processing circuitries, such as one ormore microprocessors, DSPs, ASICs, FPGAs, programmable logic circuitry,or the like, that may perform various functions and operations, such asthose described herein.

Access point 152 may comprise a device that connects to network 150 viaany of a variety of connections, such as telephone dial-up, digitalsubscriber line (DSL), or cable modem connections. In other examples,access point 152 may be coupled to network 150 through different formsof connections, including wired or wireless connections. In someexamples, access point 152 may communicate with external device 24, PD12, and/or IMD 10. Access point 152 may be co-located with patient 14(e.g., within the same room or within the same site as patient 14) ormay be remotely located from patient 14. For example, access point 152may be a home monitor located in the patient's home or is portable forcarrying with patient 14.

During operation, IMD 10 and/or PD 12 may collect, measure, and storevarious forms of diagnostic data. For example, IMD 10 and/or PD 12 maycollect EGM and motion signals, and determine different CRTconfigurations and A-V intervals. In certain cases, IMD 10 and/or PD 12may directly analyze collected diagnostic data and generate anycorresponding reports or alerts. In some cases, however, IMD 10 and/orPD 12 may send diagnostic data to external device 24, access point 152,and/or external device 142, either wirelessly or via access point 152and network 150, for remote processing and analysis. For example, IMD 10and/or PD 12 may send external device 24 data that indicates whether aloss of intrinsic AV conduction or CRT capture (fusion) was detected.External device 24 may generate reports or alerts after analyzing thedata.

In another example, IMD 10 and/or PD 12 may provide external device 142with collected EGM and motion signal data, system integrity indications,and any other relevant physiological or system data via access point 152and network 150. External device 142 includes one or more processingcircuitries 148. In some cases, external device 142 may request suchdata, and in some cases, IMD 10 and/or PD 12 may automatically orperiodically provide such data to external device 142. Upon receipt ofthe diagnostic data via input/output device 146, external device 142 cananalyze the data and generate reports or alerts upon determination thatthere may be a possible need for changed CRT parameters, or to indicateeffectiveness of CRT.

In one example, external device 142 may comprise a secure storage sitefor information that has been collected from IMD 10, PD 12, and/orexternal device 24. In this example, network 150 may comprise anInternet network; and trained professionals, such as clinicians, may usecomputing devices 144A-144N to securely access stored data on externaldevice 142. For example, the trained professionals may need to enterusernames and passwords to access the stored information on externaldevice 142. In one embodiment, external device 142 may be a MedtronicCareLink® server provided by Medtronic plc of Dublin, Ireland.

In some examples, processing circuitry and memory of one or more ofaccess point 152, server 142, or computing devices 144, e.g., processingcircuitry 148 and memory of server 142, may be configured to providesome or all the functionality ascribed to processing circuitry andmemory of IMD 10 and/or PD 12. For example, server 142 may be configuredto receive a signal from sensor 100 of PD 12 via communication with PD12 via network 150 and one or more of access point 152 and externaldevice 234. Using the signal, processing circuitry 146 may, in whole orin part, perform any of the methods described herein for adapting andevaluating CRT, including one or more of classifying beats as fusion orother beats based on the signal, controlling a timing interval or othercontrol parameter value used by IMD 10 and/or PD 12 for timing thedelivery of CRT by PD 12 based on the classification of the beats, anddetermining values of one or more metrics indicating an amount of beatsthat were fusion beats and, thus, the effectiveness of CRT. Externaldevice 142 may communicate updated values for a timing interval thatcontrols ventricular pacing pulse timing for CRT to IMD 10 and/or PD 12via network 150. In some examples, such communication to IMD 10 and/orPD 12 may be after presentation of updated values to a clinician forapproval via a computing device 144. In some examples, processingcircuitry 148 of external device 142 may determine values of one or moremetrics indicating effectiveness of CRT using the techniques describedherein, or otherwise report such values to a clinician via a computingdevice 144. In some examples, processing circuitry 148 may receivefeatures of contractions rather than the motion signal, orclassifications of beats rather than the features, and perform someportions of methods described herein using the received features orclassification information.

FIG. 9 is a flow diagram illustrating an example process for deliveringCRT through a PD 12, in accordance with one or more aspects of thisdisclosure. FIG. 8 illustrates method 200 in which CRT, such as fusionpacing, is delivered to cardiac tissue through PD 12, in communicationwith IMD 10, to address ventricular dyssynchrony present in a patient.Method 200 begins during or after IMD 10 and PD 12 are implanted intothe patient. Method 200 may use IMD 10 and PD 12 in a master-slavecommunication mode, but other communication means can be applied aswell. Additionally, method 200 is not limited to the examples in whichPD 12 is affixed to an inner wall of the left ventricle and is inwireless communication with IMD 10. Other configurations can be used.For example, the RV can undergo fusion pacing instead of the LV.Additionally, PD 12 can be placed on an outer wall of the LV and/or RV.Furthermore, although described in the context of an example in which PD12, and processing circuitry 90 of PD 12 perform a number of thefunctions illustrated in the example of FIG. 9, in other examples one ormore of these functions may be performed by one or more other devicesthat communicate with PD 12, such as IMD 10, external device 24,external device 142, or computing devices 144, e.g., by the processingcircuitry of such devices.

According to example method 200, the motion signal is received byprocessing circuitry 90 of PD 12 from sensor 100, which may be anaccelerometer or gyroscope, as examples (202). Again, as discussedherein, the motion signal may be received by processing circuitry ofother devices, including but not limited to IMD 10, external device 24,external device 142, and computing devices 144. The processing circuitryat such devices can receive the motion signal. Further, the motionsignal may be more than one motion signal, e.g., a signal for each of atwo or more axis of a multi-axis accelerometer, in some examples.

Processing circuitry 90 identifies a contraction within the motionsignal (204), and identifies features of the contraction (206). Thecontraction may be identified based on identification of certainamplitudes in an expected timing relationship to a delivered ventricularpacing pulse. Features of the contraction may include, for example, anamount or magnitude of motion or displacement in a direction, such as ina plane orthogonal to the primary axis of motion during the contraction.In some examples, the amount of motion is characterized by a sum of thedistances, at various points in time during the contraction, from apoint of origin of the contraction, or by a maximum distance or anotherone or more distances from the point of origin. Example features of acontraction signal are illustrated and described in further detail withrespect to FIG. 12.

Processing circuitry 90 compares the identified features of thecontraction to one or more criteria, which may include comparison to oneor more template values of the feature, as described herein (208). Basedon the comparison, processing circuitry 90 may classify the contractionas a fusion beat, or another beat. In the illustrated example,processing circuitry 90 characterizes the contraction as one of a fusionbeat, fully-paced beat, or intrinsic beat (210). PD 12 can assess thecardiac contraction after each pace and use this for subsequentoptimization of pace timing.

A fully-paced beat may refer to a beat in which the ventricular pacingpulse delivered to one ventricle captured the other ventricle, ratherthan fusing with the intrinsic depolarization from the other ventricle.A fully-paced beat suggests that the pacing pulse was delivered tooearly. In response to classification of a contraction as a fully-pacedbeat, processing circuitry may increase a timing interval for the CRTpacing, e.g., an A-V interval, such as an A-LV interval (212). In otherexamples, in response to classification of a contraction as afully-paced beat, processing circuitry may decrease the degree ofpre-excitation of the LV in a manner other than by increasing an A-LVinterval, such as by decreasing a V-V interval. For example, processingcircuitry may increase an A-V interval by about 10 ms, or decrease a V-Vinterval by about 10 ms.

The occurrence of an intrinsic beat after delivery of a ventricularpacing pulse for CRT (also referred to as pseudofusion), suggests thatthe pacing pulse was delivered too late. Pseudo-fusion involveselectrical activation of the cardiac tissue almost entirely throughintrinsic electrical activity with minimal or no contribution frompacing. While fusion is typically characterized by a wave complex formedby depolarization of the myocardium initiated by two different foci,commonly a non-native stimulus as from a PD and a native stimulus,pseudofusion is typically characterized by a wave complex formed bydepolarization of the myocardium initiated by a native stimulus;however, a non-native stimulus, that does not contribute todepolarization, adds a pacing artifact on top of the native wavecomplex.

Pseudofusion happens when a pacing pulse is coincident with aspontaneous QRS complex during ventricular pacing. If the tissue aroundthe electrode has already spontaneously depolarized and is in itsrefractory period, the pacing stimulus may be ineffective. Techniquesfor detecting pseudofusion beats based on the cardiac EGM have beenproposed. However, according to the techniques of this disclosure,pseudofusion beats may be detected based on one or more features of amotion signal.

In In response to classification of a contraction as an intrinsic beat,processing circuitry may decrease a timing interval for the CRT pacing,e.g., an A-V interval, such as an A-LV interval (214). In otherexamples, in response to classification of a contraction as an intrinsicbeat, processing circuitry may increase the degree of pre-excitation ofthe LV in a manner other than by decreasing an A-LV interval, such as byincreasing a V-V interval. For example, processing circuitry maydecrease an A-V interval by about 10 ms, or increase a V-V interval byabout 10 ms. In some examples, such as those in which biventricularpacing is delivered, a V-V interval may be maintained and adjusted asdescribed herein. For example, if there is delayed conduction near theLV pacing location, the paced RV activation will dominate paced LVactivation (e.g., resulting in insufficient fusion) unless the LV isgiven a “head start.” In this case, the V-V delay may be set andmodified to pre-excite the LV.

Although not illustrated in FIG. 9, processing circuitry 90 may alsodecrease the timing interval (or otherwise increase the degree ofpre-excitation of a ventricle) based on receiving an R-wave indicationfrom sensing circuitry 98 and withholding the delivery of a pacingpulse. The decrease may be the same as, or different than, the decreaseapplied when a pacing pulse is delivered by the contraction isclassified as intrinsic. For example, processing circuitry may decreasean A-V interval by about 10 ms (or increase a V-V interval by about 10ms). As illustrated by FIG. 9, processing circuitry 90 may leave thetiming interval unchanged based on classifying the contraction as afusion beat.

As illustrated in the example of FIG. 9, processing circuitry 90 mayalso update contraction data, e.g., stored in memory 92, based on theclassification of the contraction (216). The contraction data mayinclude values for one or more metrics that indicate an effectiveness ofCRT, such as a percentage or other amount of paced cardiac cycles thatresult in fusion beats. In various examples, the stored cardiaccontraction data can be at several locations including but not limitedto IMD 10, PD 12, external device 24, external device 152, and computingdevices 144A-144N. In some examples, the stored contraction data may beprovided to external device 24, external device 142, or computingdevices 144 for presentation to a user.

The example method of FIG. 9 may be performed on a beat-to-beat basis,e.g., for each cardiac cycle for which PD 12 delivers a pacing pulse, orless frequently, such as according to a X of Y cycle or X cycle everytime period schedule, or in response to an instruction from a user. Forexample, the method of FIG. 9 may be performed for one cycle out ofevery ten cycles, or for one cycle every minute.

In other examples, the method of FIG. 9 may be performed in response todetection of atrial fibrillation (AF), e.g., by IMD 10. During AF inpatients with intact AV conduction, random impulses passed through theAV node result in chaotic activation timing of the ventricles. As aresult, it is very difficult to maintain consistent pace timing andhigh-quality CRT during AF. In certain examples, IMD 10 monitors forP-waves and commands PD 12 to pace at the right times. When AF begins,IMD 10 detects the AF and could communicate to PD 12 that the patient isin AF. In response to the communication, PD 12 may begin the examplemethod of FIG. 9, including analysis of the motion signal, to maintaineffective pacing timing and capture for CRT during AF.

In examples in which AF is detected, pacing may switch from an atrialtracking mode to a non-atrial tracking mode. In a non-atrial trackingmode, the timing interval used to control the delivery of ventricularpacing is an escape interval from a paced or intrinsic ventriculardepolarization of the preceding cardiac cycle. In examples in which theatrial synchronous ventricular pacing is delivered by an intracardiac PD12 based signals from IMD 10, IMD 10 may stop sending and/or PD 12 mayignore such signals in response to AF.

The method of FIG. 9 may be performed during AF, but increasing (212) ordecreasing (214) an interval may refer to the escape interval. Anexample amount to increase or decrease the escape interval during AF isbetween about 10 ms and about 50 ms. In this scenario, processingcircuitry could also increase the escape interval by a relatively smallamount, e.g., about 10 ms, in response to detection of a fully pacedbeat (212) to avoid pacing too fast.

In some examples, medical device system 8 is a cardiac resynchronizationtherapy defibrillator (CRT-D) system that comprises an EV-ICD as IMD 10and an LV pacing device as PD 12. The EV-ICD senses P-waves andcommunicates timing of the P-wave to the LV pacing device, whichdetermines the best A-V delay for pacing. If the selected A-V delay wastoo long, the LV pacing device can sense intrinsic depolarization (e.g.,a sensed event) or can determine through the accelerometers thatpseudofusion has occurred. This implies that future A-V intervals needto be foreshortened. The LV pacing device can automatically adjusttiming intervals to achieve the longest A-V interval that avoidspseudofusion or sensed events, thus maintaining fusion. In someexamples, the pacing device could also discriminate A-V intervals thatare too short by observing a fully-paced accelerometer pattern, ratherthan an optimal fusion accelerometer pattern. This would imply the needto increase future A-V intervals. This method avoids the skipping ofoccasional paced beats, uses direct measurement of mechanicalcontraction, and can adjust A-V intervals each beat.

In another example, PD 12 may not be able to determine if effectivecapture occurs based on its LV EGM (the EGM from the pacing device isnear-field whereas far-field EGM may better assess effective capture).However, PD 12 is equipped with motion sensor 100, such as athree-dimensional accelerometer or gyroscope. The accelerometer providesfeedback about the mechanical contraction pattern. The pacing device canlearn the accelerometer pattern for an intrinsic beat and separately foran effective capture beat. During AF, if the pacing device delivers apace but sees an intrinsic accelerometer pattern, this suggests thatpseudofusion occurred and suggests the need to increase the pacing rateto overdrive AF conduction. If the pacing device senses an intrinsicventricular activation, this also suggests the need to increase thepacing rate. Alternatively, if the pacing device senses an effectivecapture beat (via accelerometers) following a pace, this suggests thateffective capture is occurring, and the future pacing rate could bedecreased a small amount, to avoid pacing too rapidly. This beat-by-beatassessment and adjustment of pacing rate will result in maximaleffective capture during AF while attempting to minimize the pacingrate.

While method 200 is described relative to PD 12 placed in the leftventricle, skilled artisans appreciate that the present disclosure canbe applied to many different embodiments in which IMD 10 is used incombination with PD 12. For example, PD 12 can be implanted within achamber of the heart or substernally/retrosternally, as described inU.S. provisional patent application Ser. No. 61/819,946 (Attorney DocketNo. C0005682.USP1) filed May 6, 2013 and entitled “IMPLANTABLE MEDICALDEVICE SYSTEM HAVING IMPLANTABLE CARDIAC DEFIBRILLATOR SYSTEM ANDSUBSTERNAL LEADLESS PACING DEVICE”, incorporated by reference in itsentirety, U.S. provisional patent application Ser. No. 61/820,024(Attorney Docket No. C0005684.USP1) filed May 6, 2013 and entitled“ANCHORING AN IMPLANTABLE MEDICAL DEVICE WITHIN A SUB STERNAL SPACE, andU.S. provisional patent application Ser. No. 61/820,014 (Attorney DocketNo. C0005685.USP1) filed May 6, 2013 and entitled “SYSTEMS AND METHODSFOR IMPLANTING A MEDICAL ELECTRICAL LEAD WITHIN A SUBSTERNAL SPACE”, allof which are incorporated by reference herein. The IMD is configured todeliver shocks to the patient without any leads implanted within thevasculature and/or heart of the patient.

FIG. 10 is a flow diagram illustrating an example process fordetermining parameter values for cardiac resynchronization therapy basedon an evaluation of one or more cardiac contraction features. Method 300may use IMD 10 and PD 12 in a master-slave communication mode, but othercommunication means can be applied as well. Additionally, similar tomethod 200, method 300 is not limited to the examples in which PD 12 isaffixed to an inner wall of the left ventricle and is in wirelesscommunication with IMD 10. Other configurations may be used as discussedwith method 200 in FIG. 9. Furthermore, although described in thecontext of an example in which PD 12 and processing circuitry 90 of PD12 perform a number of the functions illustrated in the example of FIG.10, in other examples one or more of these functions may be performed byone or more other devices that communicate with PD 12, such as IMD 10,external device 24, external device 142, or computing devices 144, e.g.,by the processing circuitry of such devices.

FIG. 10 illustrates an example method for testing a plurality of valuesof one or more CRT control parameters while monitoring the contractionsresulting from delivery of CRT according to different parameter valuesusing the techniques described herein. The value(s) of the CRT controlparameter(s) that provide desired contractions, e.g., desired values ofone or more contraction features, may be selected for ongoing deliveryof CRT therapy. The example method of FIG. 10 may be performed shortlyafter implant of a system 8, at a clinic visit, and/or automatically inresponse to a remote command or on a periodic basis.

According to the example of FIG. 10, values (such as ranges of values)for one or more parameters, including, e.g., AV-delay, VV-delay, andpacing configuration will be selected to form a CRT parameter space totest (302). Once the parameter space has been established, a parametervalue from the parameter value space will be tested (304).

Similar to step 206 in FIG. 9, processing circuitry 90 identifiesfeatures of the contraction (306). The contraction may be identifiedbased on identification of certain amplitudes in an expected timingrelationship to a delivered ventricular pacing pulse. Features of thecontraction may include, for example, an amount or magnitude of motionor displacement in a direction, such as in a plane orthogonal to theprimary axis of motion during the contraction. In some examples, theamount of motion is characterized by a sum of the distances, at variouspoints in time during the contraction, from a point of origin of thecontraction, or by a maximum distance or another one or more distancesfrom the point of origin. Example features of a contraction signal areillustrated and described in further detail with respect to FIG. 12.

Processing circuitry 90 stores feature values in association with theparameter value that has been selected from the parameter values space,as described herein (308). The values may either be stored locally, usedin closed-loop fashion to adjust the timing directly, or communicated bywire or wirelessly to some sort of control unit.

If all values have not been tested (310), the method may return to thebeginning of the method at step 304 and select the next parameter valuefrom the parameter value space. Once all of the parameters from theparameter value space have been tested (310), the parameter value withdesired contraction feature value(s), e.g., with minimal radialdisplacement, may be selected (312).

Although the example techniques of FIGS. 9 and 10 are describedprimarily in the context of a PD 12 implanted in the left-ventricle,other example medical devices may implement the techniques to evaluatethe effectiveness of CRT control parameter values. For example, anextracardiac pacemaker may deliver ventricular pacing through electrodesof one or more leads. The pacemaker may also be coupled to a motionsensor, e.g., by a lead or wirelessly, to receive a motion signal duringcardiac contraction.

FIGS. 11A and 11B show actual human three-dimensional accelerometer data(in units where 1=the gravitational force of Earth) from a pacingdevice. FIG. 11A is a plot illustrating normal sinus rhythm (NSR,intrinsic beat) looking down the systolic axis. The axes have beentransformed from the accelerometer axes of the pacing device, such thatthe primary axis of systolic contraction is now along an axis thatpoints out of the plane of the image. The plane of the image thus showsthe accelerometer components that are orthogonal to the primary systoliccontraction. FIG. 11B is a plot illustrating right ventricular (RV)pacing looking down the systolic axis. FIGS. 11A and 11B each show onecardiac cycle. In FIG. 11A, a point of origin of the contraction isidentified with reference item number 410A, a systole portion of thecardiac cycle is identified with reference item number 420A, and thediastole portion of the cardia cycle is identified with reference itemnumber 430A. In FIG. 11B, a point of origin of the contraction isidentified with reference item number 420B, the systole portion of thecardiac cycle is identified with reference item number 420B, and adiastole portion of the cardiac cycle is identified with referencenumber 430B. FIG. 11B shows that the RV paced beat has greater systolicmotion in this orthogonal plane than FIG. 11A (the intrinsic (NSR) beat)because the RV paced beat causes dyssynchronous contraction. Bydetermining an amount of motion in a direction other than the primaryaxis of systolic contraction, such as in a plane orthogonal to the axis,the degree of dyssynchrony can be estimated. In some examples, theamount of motion may be a sum of distances from an origin at variouspoints during the systole portion of the cardiac cycle. The amount ofdyssynchronous motion may be used, according to the techniques of thisdisclosure to discriminate fusion (A-V interval satisfactory) from othertypes of beats, such as intrinsic depolarization (A-V interval too long)and fully LV paced beats (A-V interval too short).

FIG. 12 is a conceptual plot of cardiac motion illustrating examplefeatures of a cardiac motion signal, in accordance with one or moreaspects of this disclosure. FIG. 12 shows a plot illustrating an exampleof one systolic portion 600 of a cardiac cycle looking down the primarysystolic axis of motion. Vectors 620A, 620B, and 620C (collectively“vectors 620”) illustrate motion or displacement from an origin of thecontraction 610 in directions other than along the primary systolicaxis, e.g., radial motion in a plane orthogonal to the primary axis,such as due to apical rocking. Although three vectors 620 areillustrated in FIG. 12, processing circuitry may determine more or fewervectors 620 to determine a feature of a cardiac contraction as describedherein. FIG. 12 shows the beat having a large systolic motion with avector 620A from origin of the contraction 610. The amount of motion ina direction other than the primary axis of systolic contraction, e.g.,the maximum, sum, or mean of vectors 620, may be used to estimate thedegree of dyssynchrony, e.g., as a value of a feature of a cardiaccontraction. Vectors, such as vectors 620B and 620C, with shortdistances to origin of the contraction 610 may be used to indicate theabsence of a dyssynchronous contraction. While vectors, such as vector620A, with large distances origin of the contraction 610 may indicate adyssynchronous contraction.

The disclosure also contemplates computer-readable storage mediacomprising instructions to cause a processor to perform any of thefunctions and techniques described herein. The computer-readable storagemedia may take the example form of any volatile, non-volatile, magnetic,optical, or electrical media, such as a RAM, ROM, NVRAM, EEPROM, orflash memory. The computer-readable storage media may be referred to asnon-transitory. A programmer, such as patient programmer or clinicianprogrammer, or other computing device may also contain a more portableremovable memory type to enable easy data transfer or offline dataanalysis.

In addition, it should be noted that system described herein may not belimited to treatment of a human patient. In alternative examples, thesystem may be implemented in non-human patients, e.g., primates,canines, equines, pigs, and felines. These other animals may undergoclinical or research therapies that may benefit from the subject matterof this disclosure. The techniques described in this disclosure,including those attributed to IMD 10, PD 12, external device 24, andvarious constituent components, may be implemented, at least in part, inhardware, software, firmware or any combination thereof. For example,various aspects of the techniques may be implemented within one or moreprocessors, including one or more microprocessors, DSPs, ASICs, FPGAs,or any other equivalent integrated or discrete logic circuitry, as wellas any combinations of such components, embodied in programmers, such asphysician or patient programmers, stimulators, remote servers, or otherdevices. The term “processor” or “processing circuitry” may generallyrefer to any of the foregoing logic circuitry, alone or in combinationwith other logic circuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. For example, any of thetechniques or processes described herein may be performed within onedevice or at least partially distributed amongst two or more devices,such as between IMD 10, PD 12, external device 24. In addition, any ofthe described units, modules or components may be implemented togetheror separately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

The techniques described in this disclosure may also be embodied orencoded in an article of manufacture including a computer-readablestorage medium encoded with instructions. Instructions embedded orencoded in an article of manufacture including a computer-readablestorage medium encoded, may cause one or more programmable processors,or other processors, to implement one or more of the techniquesdescribed herein, such as when instructions included or encoded in thecomputer-readable storage medium are executed by the one or moreprocessors. Example computer-readable storage media may include randomaccess memory (RAM), read only memory (ROM), programmable read onlymemory (PROM), erasable programmable read only memory (EPROM),electronically erasable programmable read only memory (EEPROM), flashmemory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, acassette, magnetic media, optical media, or any other computer readablestorage devices or tangible computer readable media.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A system for determining the effectiveness ofcardiac resynchronization therapy (CRT), the system comprising: a pacingdevice configured to be implanted within a patient, the pacing devicecomprising: a plurality of electrodes; signal generation circuitryconfigured to deliver ventricular pacing pulses for the cardiacresynchronization via the plurality of electrodes; and a sensorconfigured to produce a signal that indicates mechanical activity of theheart; processing circuitry configured to: for each of a plurality ofcardiac cycles during which the pacing device delivered one of theventricular pacing pulses, identify one or more features of a cardiaccontraction within the signal during the cardiac cycle; for each of theplurality of cardiac cycles, determine whether the cardiac contractionis a fusion beat based on the one or more features; and determine avalue of at least one metric indicating the effectiveness of the CRTbased on the determinations of whether the cardiac contractions arefusion beats; and a user interface configured to present the value ofthe at least one metric to a user.
 2. The system of claim 1, wherein thepacing device comprises a housing configured for implantation on orwithin the heart, wherein at least one of the signal generationcircuitry and the sensor are within the housing.
 3. The system of claim1, wherein the ventricular pacing is delivered to a left ventricle. 4.The system of claim 1, wherein the sensor comprises a three-dimensionalaccelerometer.
 5. The system of claim 1, wherein the one or morefeatures of the cardiac contraction within the signal comprise one ormore of a slope of the signal during the cardiac contraction, anamplitude of the signal during the contraction, a maximum of the signalduring the cardiac contraction, a minimum of the signal during thecardiac contraction, a ratio of the maximum and the minimum during thecardiac contraction, or a polarity of the signal during the cardiaccontraction.
 6. The system of claim 1, wherein the one or more featuresof the cardiac contraction within the signal comprise one or more of anamount or a direction of motion relative to a point of origin during thecontraction.
 7. The system of claim 6, wherein the one or more featuresof the cardiac contraction within the signal comprise an amount ofmotion in at least one direction other than the primary axis of motionduring the cardiac contraction.
 8. The system of claim 6, wherein theone or more features of the cardiac contraction within the signalcomprise an amount of motion in at least one plane orthogonal to theprimary axis of motion during the cardiac contraction.
 9. The system ofclaim 1, wherein the one or more features of the cardiac contractionwithin the signal comprise one or more values resulting from comparisonof the signal during the cardiac contraction to one or more templates.10. The system of claim 9, wherein the one or more templates compriseone or more of an intrinsic beat template, a fully-paced beat template,or a fusion beat template.
 11. The system of claim 1, wherein the valueof at least one of the metrics indicates an amount of the contractionsthat were fusion beats.
 12. The system of claim 11, wherein the amountis a percentage of the contractions.
 13. The system of claims 1, whereinthe processing circuitry comprises processing circuity within the pacingdevice.
 14. The system of claim 1, further comprising anotherimplantable medical device, wherein the other implantable medical deviceis configured to: detect an atrial event; and transmit a signal to thepacing device to control the pacing device to deliver the ventricularpacing pulse in response to the detection of the atrial event, whereinthe processing circuitry comprises processing circuitry of the otherimplantable medical device.
 15. The system of claim 1, furthercomprising an external device configured to communicate with the pacingdevice, the external device comprising the user interface, wherein theprocessing circuitry comprises processing circuitry of the externaldevice.
 16. A method for determining the effectiveness of cardiacresynchronization therapy (CRT) by a pacing device, the methodcomprising, by processing circuitry of a medical device systemcomprising the pacing device: receiving a signal from a sensor of thepacing device, the signal indicating mechanical activity of a heart; foreach of a plurality of cardiac cycles during which the pacing devicedelivered a ventricular pacing pulse for the CRT, identifying one ormore features of a cardiac contraction within the signal during thecardiac cycle; for each of the plurality of cardiac cycles, determiningwhether the cardiac contraction is a fusion beat based on the one ormore features; determining a value of at least one metric indicating theeffectiveness of the CRT based on the determinations of whether thecardiac contractions are fusion beats; and presenting the value of theat least one metric to a user via a user interface.
 17. The method ofclaim 16, wherein the pacing device comprises a housing configured forimplantation on or within the heart, wherein the sensor is within thehousing.
 18. The method of claim 16, wherein the ventricular pacingpulses are delivered to a left ventricle.
 19. The method of claim 16,wherein the sensor comprises a three-dimensional accelerometer.
 20. Themethod of claim 16, wherein the one or more features of the cardiaccontraction comprise one or more of a slope of the signal during thecardiac contraction, an amplitude of the signal during the contraction,a maximum of the signal during the cardiac contraction, a minimum of thesignal during the cardiac contraction, a ratio of the maximum and theminimum during the cardiac contraction, or a polarity of the signalduring the cardiac contraction.
 21. The method of claim 16, wherein theone or more features of the cardiac contraction within the signalcomprise one or more of an amount or a direction of motion relative to apoint of origin during the contraction.
 22. The method of claim 21,wherein the one or more features of the cardiac contraction within thesignal comprise an amount of motion in at least one direction other thanthe primary axis of motion during the cardiac contraction.
 23. Themethod of claim 21, wherein the one or more features of the cardiaccontraction within the signal comprise an amount of motion in at leastone plane orthogonal to the primary axis of motion during the cardiaccontraction.
 24. The method of claim 16, wherein the one or morefeatures of the cardiac contraction within the signal comprise one ormore values resulting from comparison of the signal during the cardiaccontraction to one or more templates.
 25. The method of claim 24,wherein the one or more templates comprise one or more of an intrinsicbeat template, a fully-paced beat template, or a fusion beat template.26. The method of claim 16, wherein the value of at least one of themetrics indicates an amount of the contractions that were fusion beats.27. The method of claim 26, wherein the amount is a percentage of thecontractions.
 28. A system comprising: means for receiving a signal froma sensor of the pacing device, the signal indicating mechanical activityof a heart; for each of a plurality of cardiac cycles during which thepacing device delivered a ventricular pacing pulse for the CRT, meansfor identifying one or more features of a cardiac contraction within thesignal during the cardiac cycle; for each of the plurality of cardiaccycles, means for determining whether the cardiac contraction is afusion beat based on the one or more features; means for determining avalue of at least one metric indicating the effectiveness of the CRTbased on the determinations of whether the cardiac contractions arefusion beats; and means for presenting the value of the at least onemetric to a user via a user interface.
 29. A computer-readable storagemedium comprising instructions that, when executed by processingcircuitry of a medical device system, cause the processing circuitry to:receive a signal from a sensor of the pacing device, the signalindicating mechanical activity of a heart; for each of a plurality ofcardiac cycles during which the pacing device delivered a ventricularpacing pulse for the CRT, identify one or more features of a cardiaccontraction within the signal during the cardiac cycle; for each of theplurality of cardiac cycles, determine whether the cardiac contractionis a fusion beat based on the one or more features; determine a value ofat least one metric indicating the effectiveness of the CRT based on thedeterminations of whether the cardiac contractions are fusion beats; andpresent the value of the at least one metric to a user via a userinterface.
 30. A system for determining the effectiveness of cardiacresynchronization therapy (CRT), the system comprising: a pacing deviceconfigured to be implanted within a patient, the pacing devicecomprising: a plurality of electrodes; a signal generation circuitryconfigured to deliver ventricular pacing pulses for the cardiacresynchronization via the plurality of electrodes to a left ventricle; athree-dimensional accelerometer configured to produce a signal thatindicates mechanical activity of the heart; and a housing configured forimplantation on or within the heart, wherein the signal generationcircuitry and the sensor are within the housing; processing circuitryconfigured to: for each of a plurality of cardiac cycles during whichthe pacing device delivered one of the ventricular pacing pulses,identify one or more features of a cardiac contraction within the signalduring the cardiac cycle; for each of the plurality of cardiac cycles,determine whether the cardiac contraction is a fusion beat based on theone or more features; and determine a value of at least one metricindicating the effectiveness of the CRT based on the determinations ofwhether the cardiac contractions are fusion beats; and an externaldevice configured to communicate with the pacing device, the externaldevice comprising a user interface configured to present the value ofthe at least one metric to a user, wherein the processing circuitrycomprises processing circuitry of the external device and processingcircuitry of pacing device, and wherein the one or more features of thecardiac contraction within the signal comprise an amount of motion in atleast one direction other than the primary axis of motion during thecardiac contraction.